After fishing as both when I was doing tournaments….in my prechild days. 
Here are some things to consider….
If you can’t back a trailer in extremely well make sure the boater knows this and can have someone else lined up to help him, holding things up at a landing tournament morning is extremely frustrating for everyone.
Treat the boaters and their boats with the upmost respect. Don’t make a mess, don’t step on the rails and if you are unsure where to step to get in just ask. Ask where the boater wants you to put your gear.
Ask your boater the night before if there is anything you should or shouldn’t bring. Life jacket, snacks, water, whatever. Bring just the necessities.
When I fished as a coangler I would bring chewy granola bars for a snack during the day, they were quick to eat and no mess. I always had a couple extra and offered them up when ever I had one. Always water to drink, if it spilled no biggie, I’ve seen coanglers spill coffee/pop, yes it cleans up but how would you feel if someone hopped in your truck and spilled coffee all over the place. I actually had a co-angler one time that brought a big bag and a cooler and then spent more time eating than fishing during the day.
Chipping in for gas and bait is really appreciated, it’s expensive to prefish and participate in tournaments. If for some reason you choose not to chip in, just walk away, I actually had a co-angler tell me he would have chipped in for gas but my check was going to be bigger than his so he didn’t feel he needed to.
If you are using his gear make sure it is set up for you before you leave the dock in the morning, switching reel handles and stuff like that. Switch them back at the end of the day.
As the co angler….I let my boaters know exactly what my level of fishing was, don’t let pride get in your way be honest. If he has an option that works to your strengths it is that much better for both of you.
As the boater I asked what my coangler was good at and how they felt about the kind of fishing I was planning on for the day. By that I was able to provide the needed guidance to help them/us out.
It is okay to offer suggestions but don’t push the issue, I had a co-angler that questioned me all morning and finally I asked him what the odds were of us catching fish on “his spot”(he prefished by himself) and his reply was 50/50….we were on Leech, I told him you could throw a dart and have a 50/50 shot at catching a walleye on Leech which at least kept him quiet.
The biggest thing for me was if you sign up to fish a tourny take it seriously, focus on fishing, having a fun day on the water and do your best that is all a boater can really ask.