There are tons of obsolete tickets crowding this system. And you've ironically added to that problem by posting this. But as already stated, this site needs some *heavy* moderation. Get help from the community if neccessary, but please get it sorted. If you look through a few bugs, odds are huge you'll come across something that should have been closed.
Sometimes mods or devs give feedback that a bug is fixed, but leave a comment for the initial reporter to confirm and close. That is frankly pointless and stupid. If it's fixed, close the ticket, and if anyone still has the problem they can post a new one.
And look at this; http://feedback.arma3.com/view.php?id=13739. Why isn't stuff like this just removed. Even if you are entertaining the possibility of dealing with it, it would be so far down the huge stack of other issues priority-wise, that you might as well remove it from the system completely for now.
And don't even get me started on the duplicates...

I honestly don't understand how BIS is able to use this system for anything useful. It's like a wasteland in here, full of dead tickets and orphaned requests. Surely it would be good for the devs as much as anyone if this place was better moderated?

Naturally. I just think it's good practice to give feedback on the dev branch, otherwise the developers will be fixing issues blindly until they roll everything into stable.

I updated my report. Latest dev branch version released today seems to solve the problem. Also tested and confirmed your problem in current 1.10, and it seems fixed as well.
Can you confirm and report back if resolved?

Probably closely related to my report: 0016993

Improvements seen in the latest dev branch. Hoping a solution will make it into 1.11.

Issue seems fixed in 1.12 stable. Rogerx, can you double check that you are on 1.12, and check if the issue occurs when tested in the editor? If your issue isn't replicatable there, I suspect it might be related to a particular mission, or be specific to MP in some way?

1.11.115366 seems to solve this nicely. Can enter water from shore or boat, with any combination of pistol, rifle and no weapon, with or without a wetsuit.
I think this one can be considered resolved.

Improved. Tested in 1.11.115293. I have not been able to reproduce the issue with the rifles under that version. Now however, an identical problem arises with a standard pistol, tested only with the P07 so far. And it occurs both when disembarking and when wading into the water from shore with the pistol in hand. Problem still appears to be associated with wetsuits, as I have not identified any other uniform or lack thereof that will trigger it.

Possible patches and changelog notes associated with fix and "regression":
*1.11.115121: "Weapon remains selected for divers, it just cannot be fired (unless underwater capable)"
*1.11.115293: "Player without swimming suit doesn't get messed up when entering water with weapon"

I feel that the perception of speed is fine. If anything, it sometimes feels like it's going faster than it says it is. In reality, you won't have very good perception of speed in a boat, unless you're very close to shore. To some extent that might translate to the game as well, making it feel slow when you're far from shore.

As far as the actual speeds are concerned, I've only ever noticed the assault boat standing out as a bit sluggish. Posting a Wikipedia link about RIBs is a bit off though. The boat in the game seems to be a standard inflatable with an inflatable keel, as used by many armed forces. These won't perform nearly as well as a RIB. A small 4-5 metre inflatable is very sensitive to weather conditions and load, but is also very sensitive to engine power.

I assume the "500 fivestrike" label on the engine is a bit of a developer joke, like the "FTW" badges on certain cars in the game. However, even with a 25 hp engine at full throttle, a boat like that should be able to do maybe 20 knots. That's 37 kph, while the ingame boat maxes out at somewhere around 31. And for boats in actual military use, you can expect at least a 40-60 hp engine.

As a final note, some of the boats have a very small difference between cruise speed and turbo/max/full speed. Again the assault boat is perhaps the most noticeable, with the difference being around 3-4 kph. The only thing that really goes up when you max out the assault boat is engine noise.
And speaking of "fivestrike". Assuming the engine is supposed to be a modern four-stroke, it seems just a little bit noisy, but I might just be nitpicking now.

/upvoted

And my thoughts were the same as lavbo0321 on this matter. Fins should really be a separate item, like helmets and goggles. This might require much more work with regards to modelling, gui/engine changes, compatibility with modding and such, but would ultimately be the best solution.

Doing this would of course also hurt the usefulness of wetsuits besides the cosmetic/realism effects. I guess speed/mobility could be affected by the wetsuit. Or the ability to use underwater weapons effectively, as is currently the case. But that's probably beyond the scope of this ticket anyway.

This is currently the case with every boat in the game. It happens if you hold down the throttle and leave the drivers position, regardless of where you move. If you hold down the throttle while ejecting from the boat, it will continue forward on it's own.

I don't actually see this as a bug, just a bit lacking in polish. It's really quite convenient. For large boats with inboard motors, you'd expect the throttle to stay where you put it. It's not a car, it doesn't have a pedal that needs to be held down. Every boat engine has "cruise control". This is also the case with most small outboards and tiller controls.
All outboards and many inboard engine controls, will however have a dead man's switch, stopping the engine if the operator falls overboard or otherwise leaves the controls.

Personally, what I want is analog speed controls for vehicles, and I'll probably be making a feature request for that at some point. Though I'm waiting to see if the analog steering that was recently put in gets a follow-up.