I think it just proves that your bike really did make it "all the way" and not just "almost all the way". If you do it, I'd follow these guidelines.
(1) Take off the panniers to reduce weight.
(2) Take off your shoes and socks to keep them from getting wet.
(3) Find someone to take the picture because the self-timer cannot judge the waves. Make sure the photographer is aware of the plan.
(4) Carry your bike across the beach to avoid getting sand into the chain and all over everything.
(5) Stand at the high end of the wet sand.
(6) Pick a wave that looks like it will come up far enough to touch the wheel but not so far as to soak your chain and signal your photographer.
(7) As the wave hits you feet, set the bike down and have the photographer snap the picture (can be a timing problem with most digital cameras as they often have a time lag--it may take a few tries to get it right so you need a patient photographer).
(8) Pick your bike up and carry it back across the beach.
If you don't do this, you may find the need to clean your bike and relube your chain before you can take off.
All in all, it sounds like it might be more trouble than it's worth. Best bet would be to find a boat ramp or something similar. I have seen people remove just one wheel from the bike and dip that to simplify the process, but that seems to dilute the symbolic meaning a lot--you might as well just go dip your feet.