Just a few ideas for trip sharing. Some of these items will seem laughably obvious to some of the salty vets that frequent this forum, but to others who have never owned a boat or never spent a lot of time on or near them some of this stuff helps guarantee you'll get to go back out again.
All too often hear boat owners who felt slighted by a guest who didn't help out with fuel costs. While every boat is different, a 40 mile day could have burned as much or more then a gallon every 2 miles. So that's 20 gallons and at the price of $3.79 I last fueled up at in the water, it really adds up. A fuel efficient 18 footer will be far better on gas. 2 miles per gallon is a decent number for a 24 footer. Some owners really don't care about exact numbers, so it may be hard to tell what was burned. And they often won't say or for that matter don't know. I generally take a quick look at the odometer on the boat in the AM so I can guess what was burned at the end of the day. When I'm a guest I generally try to cover the entire fuel bill. Even if the captain tries to turn down the $$ I usually insist.
Bottom line is helping or covering fuel costs is standard procedure when you're a guest. I wouldn't go so far as to say it's expected, but it is for the most part. It's a very good idea to get the rules regarding fuel set before leaving the dock. This way there's no surprises for anyone at the end of the day. Many owners are happy just to split the cost, but as I said actually covering the bill isn't out of place.
No black sole shoes. Barefoot or something that doesn't mark the decks.
Drinks and food. Better bring them. Many boat owners have at least a surplus on board and will try to offer them, but I like to bring my own. You can never have too much water on board. Another reason to have a lot is you never know when awesome fishing will develop. I've had countless 4 hour trips turn into sun up to sundown fishing days.
Sunblock. Bring it. 'Nuff said. Foul weather gear too. Especially in something like an 18 footer, when the seabreeze kicks up in the afternoon you'll want it. I ALWAYS bring my foul weather gear no matter what the forecast or how big the boat. Random rainstorms can really chill you off even in the dead of summer too. Consider it safety gear.
Seasickness. Dramamine the night before and in the morning if you're concerned about it. Scopolamine (prescription, but awesome I've heard) is another great option.
Knots. If you don't know them, don't tie them. Or, as the saying goes, "if you don't know how to tie a knot, at least tie a lot."
Seriously, if the boat is at a slip let the captain finish tying the boat up at the dock unless you're very confident in your seamanship. Knowing some basic knots never hurts. The bowline and of course the cleat hitch are two key knots.
Safety. It's commonly forgotten, but knowing where life preservers are stashed isn't a bad idea. $hit happens...
End of the day. Get the money issue out of the way and offer to help out with washing the boat down. Some boat owners are REAL particular so they may not want help, but I always ask.
Feel free to add some more ideas guys. There's probably something I'm forgetting...