I've jotted down some things that I found handy whenever I've gone out on the boat for the day:

Extra clothes: it may be 80 degrees on land, but the air on the water gets chilly

Raincoat/Poncho...you never know!

Cooler with water, soda, juice (some boats have a cooler you can use)

Some snacks for munching

Sandwiches

Chapstick!

Sunblock

Hat with a visor

Sunglasses

Hand santizer/wipes

CAMERA!!!!  LOTS OF FILM!!!  CHARGE YOUR DIGITAL BATTERIES!!!

Don't forget to tip your Captain!