Saying that boats should approach docks at a slower speed is not that easy.
Some areas are very shallow and the boats must be on plane to reach the deeper water near the docks.(unless you want a lot more dredging)
Keeping the Boats way off shore is also not clear cut as there are coral heads all over and this is what determins the boat lanes.

Swimmers must be made aware that there is a lot of boat traffic, Captians must be made aware there are a lot of swimmers.
This problem is not unique to Belize, there are more boating accidents in the US every year. So for some to say they would not come or come back to A/C is ridiculous.

Visitors should be told, by signs,their hotel or whoever, that it is dangerous to swim far from the docks or beach, even after this terrible accident I have seen people swimming in the middle of the boat lanes.
We can implement as many rules and laws as we like but if people don't follow them the same problems will occur.
Maybe it is time to have a small patrol boat to warn swimmers and or Boat Captians if they are seen doing something dangerous.

If you outlaw guns, only outlaws will have guns.
A bullet fired,cannot be called back.
A.C.G.C.