Aluminum allow for marine application

I'm thinking about welding up some aluminum trusses for use as a pier extension (fresh water). The extension would only be in use during the summer months and removed once the weather starts to turn in late September.

Am I better off with round or square tube for the truss supports? Should I set the truss supports at a 45 degree angle to the top and bottom members, or would a steeper angle be better?

Also, would 6063 be the best alloy for this?

TIA for any advice...

Round "pipe" is the cheapest shape to buy.

I use 6063 when I want to make curved things.

Weld it up using 5356 filler for better corrosion resistance.