Congratulations on the sailboat getaway.
Preferably go for a sealed box design which will be much easier to accommodate. It is, on a boat, quite acceptable to lay the speaker over on its side and not concern yourself with imaging or soundstage. Even with a speaker on its side it will still be difficult to find one no more than 145mm wide. With a sub helping out you get an immersive sound.
A sealed box will provide less bass but have a low frequency roll-off response that will work in your favour. Remember there will be significant room gain providing a bass lift. It is for this reason sealed is better.
Seriously consider a sub from the get go, it makes a huge differece and you mention you have space for one.
You also mention the auto side which I recommend for amplification. 3-channel car sound amps will do the job very nicely. A number of posts express concern with a marine environment which you can handily ignore.
Wondering about this advice? : Well I lived on my 40ft steel Peter Strong for 32 years and at times during change of watch had waves over the poop and into the main saloon of my center cockpit layout. The speakers, a pair of Boston car units mounted up high and an Xtant amp mounted in a locker never failed and had no problem with rust. The chassis of the amp was well connected to the steel hull to inhibit electrolysis. I assume you have a fibreglass hull which means you should run a cable to your nearest sacrificial anode from the amp even though you said you will be sailing in fresh water.
I can describe how to join or connect the power cables etc. that can survive being permanently submerged in salt water as in bilge pumps. Won't go into details here but you can PM me for info. Happy to help.
The performance I got from mine was some of the best I have heard. Just a lot of fun. Running the genny or using an inverter is complicating things, so I suggest hooking up directly to 12V battery. Run from the common terminal on your battery change-over switch.