I can only load so many vehicles in an area before the game becomes unstable. There may be boat spawns there but they don't spawn because there are already too many vehicles in the area.
These are the vehicle spawns in that area:

I can see a few I reckon would be boats. I _can_ put a flag on a vehicle spawn to force it to spawn regardless of the vehicle density - I will look into this.
These are all the boat spawns (that are commented as such - there are bound to be more):

And that area:

I will look at adding more and forcing them to spawn.