At the current moment the commander's GMG on the speedboats is nearly useless.
The stabilization is absolutely terrible irrelevant if the boat is moving or there are waves.
When going to 300m or more, the weapon becomes so inaccurate a single rifleman could be considered more effective. When going to closer ranges the boat will become the easiest fragile target for any AT soldiers due to its fragility and its incapability to return fire accurately.
What i suggest to be done is greatly improve the stabilization of the GMG on all speedboats along with accuracy. This would make the vehicle decently usable and efficient worth using.
Stabilization technology exists and is implimented in weapon systems no matter land or water. Check out Trackfire Remote weapon stations.
Demonstration video: https://www.youtube.com/watch?v=uPmnxZFr4NI