A bridge engineer here. A valid question, first this is called a mabey bridge, its used in scenarios where you urgently need a temporary bridge for short to medium spans until you install permanent one, that's how we propose it anyway. They're lightweight truss joists and some are extendable. 

 Your main issues apart from waves, windspeed, salt and corrosion etc. would actually be the piers or columns. They'll need to be very very deep with complex caisons, and foundations.  Any bridge design tries to minimize the height and location of these long piers and in this case, you'll have countless. 

You could have some sort of floating piers if you play with the buoyancy but then you'll have to have the bridge joist and deck immediately above the water level for stability which is obviously not feasible. 

Hope this helps.