If you don’t have comfortable battery storage/solar generation to run a 5W Raspberry Pi 24/7, you should not be considering OpenHAB on your boat. Power usage is key, for sure – any Watt that can be trimmed, should be – but if a few watts/H is a problem then no automation system will work for you.
I am heavily automated with a lot of stuff that I can turn off if I must, but my ‘house computer’ (OpenHAB, plus internet routing+VPN, NAS, and some other things) is a OCdroid-XU4 that burns about 30W (disks, LTE, etc. The bare board itself, doing nothing, only runs about 8-9W). My inverter kills 10W even if nothing is using AC.
If I’m not running my main PC (Yes, I have 16U of 19" rackspace in my RV, don’t judge me :D), and merely on the bare minimum, I run about 60-70W. I have 800Ah@12V of LiFEpo battery and can basically run forever on minimum, no matter how cloudy the day. Minimum is no fun, I like lights and surround sound and my big TVs… but OpenHAB can handle it. And OpenHAB 3 is so much more efficient than OpenHAB 2.