Soap, water, leaf blower or shop vac on blow to dry, then re-lube.

Try not to submerge the bike in the water...you don't want salt water in your hubs, brake lines, BB etc. If you do, soap, water, dry, re-pack.

If it's a rental bike, you can skip all of this and simply watch for sharks as you try and pedal in the surf. :-).