ground wire to neutral in a boathouse
This is a special situation. 

Don't fool around with grounds around water

Electrical drownings are some of the most tragic fatalities around electrical.  Current that is nowhere near lethal can stun a person to where they cannot keep their head above water, and they drown.   Far worse, when someone goes to rescue them, they too suffer the same fate, like these four girls and nearly, several more rescuers.

Or it may be mistaken for a plain drowning, and only discovered after an unusual string of drownings (and more).

In fact, the death of more than one person (victim and rescuer) is the hallmark of an electrical drowning. 

There are literally hundreds of these incidents. 

Using Earth as a "ground wire" actually causes this

The whole point of the ground wire is to return fault current back to source.  Current wants to return to source, not earth, and artificial current's source is your electrical service (i.e. neutral).  If current leaks, you want to efficiently return it via the grounding wires back to the main panel and back to the power company.

Current flows down every available path in proportion to its conductance (1/resistance) - you can't do anything about that, but you can provide an extremely high-conductance path so that proportionately very little (almost no) current flows any other way. That's what a ground wire does. 

Anyway, earth - even wet earth - is a terrible conductor.  That's why we go to all the trouble to mine copper :) It's low enough conductance that if it's in parallel with a human, some fraction of total current will flow through the human.  And that will drown them. 

So we definitely do not want to go to earth as our current return, because that will cause current to try to return via earth, water and anyone in the way. 

Tying ground and neutral causes a single point of failure

If your neutral-ground wire breaks, and that is a common enough problem, it means that normal current tries to return to neutral (but can't) so it floats the neutral up toward 120V.  Neutral is tied to ground at the boathouse, so ground also floats to 120V.  And ground is tied to equipment in contact with water, so it also floats the water and earth up to 120V.   

Now if you have separate neutral and ground back to the house, it may seem clever to bond them to halve your chances of a wire break.  Actually, not so much.  Many things which would sever neutral would also sever ground, and you're back to this situation.   

Whereas  if neutral and ground are kept separate and severed, the equipment just won't work.  It will not electrify the grounds/water unless it also has a ground fault.  So it requires a double failure to have a problem.  It's true that second failure could happen first and lurk until the first failure happens, but if you are using GFCI protection, it won't be likely.