If AI assault boats are given a waypoint to move towards land, disembark and then move inland the boats' actions are unpredictable.
On occasion the boat may move up close to the shore and then stop. On others the boat may move towards the land and, while still offshore, start going around in circles rather than landing. On other occasions the process might work perfectly. These different results will often occur through different replaying of the same battles.
In the same way, if infantry are given a waypoint to board an assault boat and move out to sea, they may approach the shore but go no further, stand by the boats and refuse to get in, take an extremely long time to get in or work perfectly. Again, different results for the same battles.
This may also be the case with divers swimming to a location, exiting the water and moving inland and vice-versa, but I have been unable to track this.