Funny... NOTHING IN THE WORLD wrong with doing it yourself.... there are several things you need to know and the manufacturer's website should tell you most of it such as how to run the water lines such that they don't airlock...
Typically the return air is a square louvered vent often with provision for a filter close to the unit and there is generally no duct... it simply allows air into the compartment housing the unit... Sound doesn't go around corners well so if that louver doesn't face the unit and better yet if you have a baffle all the better.
Duct's are VERY simple and the hardest part is gaining access to route them.... In a typical aft cab cruiser (mine specifically) the duct is routed down one side and gets smaller as it passes junctions... In my 28'er I have one register on the galley counter behind the stove (HATE that location where a "pro" put it) one in the head below the sink, and one in the back wall of the closet facing into the aft cabin... again don't like that location either as it is too directly 'in your face' .... I also have another duct run down the other side to a large register under the dinette.. This is the best located register on the boat... I would have put more effort into the install... I would have put 2 very small vents in the v-berth pointing up on either side. I would have put one or two in the wall behind the galley AWAY from the stove since the last thing you want to do is blow that heat/steam into the cabin instead of letting it rise to the open hatch.... I would have put 1 vent or 2 smaller vents in the outside wall of the head so they would be above most shower spray... I would have put the aft vents below the bed such that they directed across the area instead of directly at me while sleeping.
Vents that are a bit big can be closed down but if they are too small your SOL
You CAN route your ducts and vents in the open in your boat to try out the layout before you commit and cut holes....
If you are not completely sure of the access in an area decide on an alternate plane BEFORE you cut.... IE would an electrical outlet, light, speaker, or??? work here.... don't cut the hole too big for the alternate until you are sure....
I for one would MUCH MUCH MUCH rather design and install my own system than deal with what a "pro" thinks is good..
Last, don't don't don't underestimate the size of your unit.... consult with the manufacturer of the unit... Study your boat VERY close.... do you have lots of glass? a drafty cabin door? kids going in and out constantly?
My boat came to me with all ductwork in place but the unit and pump removed... I got a smokin deal on a used marineair 10k unit w reverse cycle and installed it... the unit works great up to 80* outside... 85 and sunny it will almost keep up if the boat is chilled at night and it starts to get a bit stuffy by lunchtime even closed up and running constantly... 90*+ day forget it.... 10am and you're getting cooked... My boat seals up well and has very little window... I have reflectix insulation under canvas covers on the hatches..... I am going to install a 16k unit and I expect that one to have trouble keeping the boat cool while the stove is in use.
