JCN,
Beams came in at $1,310.00 each (those were the 40')
It depends on the end fixity of the beam in a solid setting, and how solid that foundation is.
If you used I beams, I think you could bore in and pour 4 concrete piers, use 24' beams, with 9' anchored on the piers and 15' hanging over the water.
I think that would be reasonable, but there is a lot of number crunching involved.
I really like the idea, but like I said, It's probably not the best candidate project on a DIY level, if you are not familiar with the engineering involved and want to make it safe for everyone you love.
