I ditto whoever said leave the transmission fluid alone. Just check the level and be done with it on a timely basis.
From experience..... pull the hose off the exhaust mixing tube every 4-6 months and use a wire to clean it out,,,, even if there is no over-heating problems. I think it would be a good preventative measure. The increase in back pressure due to a narrowing exhaust tube will cause several things to fail and when it does you'll be chasing your tail to figure out why.
Keep your diesel tank full OR install a device called
http://h2out.com/ which will keep humidity from building up in your fuel tank. Once water starts to build up in the bottom of your fuel tank it's just a matter of time before a small hole/leak will form. Drain the bottom of your racor filter bulb every time you change racor filters.
If your boat stays in salt water, rig up T switch on your engine water intake so you can hook up a water hose. Flush your engine from time to time, maybe just as you head home for a week or so. Let that fresh water sit in your engine to keep the salt deposits down. If you have AC, do the same to that system at least every 6 months to a year to kill the little critters that build up.
Everything else is in the maintenance manual as stated in the previous posts.