I looked into the sea ice machines and they are mainly intended for large commercial operations and are very expensive and big and heavy.

I was thinking of installing a smaller used FW ice maker(just the headportion) inside the top portion of fishbox (I have a large fishbox). And when I am dockside and have a hose and electric power it can make cubes and let them fall directly into the fish box. Otherwise it will be off. I know I will have to protect it from fish gunk but this might work.

I hate hauling large bags of ice around and my fishbox needs at least 150# of cubes to have any meaningful amount of ice in it. Half the time I need it at 10:00 at night and I am making calls to friends who run resturants and tackle stores to raid their machine. The box can take probably 400# easy. (and I have 2 boxes this size in deck + two big fishbags that can hold more if needed.)

I was going to go down and talk to my local refrig/AC guy who I know rebuilds and resells ice machines and ask him about installing a water cooled unit on board.

What do you think? Have I lost my mind or is this a good idea?

Sounds like a potentially "good" idea and getting a refurbished unit seems like the way to go. Talk to Raider, I know he has 2 ice machines @home and hauls ice.

Ice machines can be nothing but trouble.....especially on a boat near salt water? No ice plants on the Vineyard? I get ice from a local place that supplies ice to all the comm canteen trucks, very reasonable low $. 2nd option is the Ice Plant but $$ are 3-4X. 3rd option is the 10lb bags. These options still mean hauling ice but no where near headaches of having your own ice plant (on board).

Good idea... Fished on a Cabo years ago that made salt ice. Had plenty of ice in the cockpit fish box. Never had a problem with it. Nice feature. Don't know the economics.

a couple friends have their own commercial ice machines at their house and haul it. I could go that route you can get a used refurb machine for about a grand. The head alone even less I would think. A 50# bag cost about 17 bucks here. I normally need 2 or three and and have to carry this around and load it on the boat. A real PIA. The payoff is 30 trips. Probably less then one season (not including the cost to run the machine) but it would be great to go down to the boat and open the fishbox and see it 1/2 full of ice instead of carrying ice around in my house/truck/and loading on boat. Also for the long offshore trips where you need ice to the gills you just let her run a few days.
Also the cost to run the machine on the boat is essentially zero. Power and watter comes from the dock...already paid for. :)

My friends give it to me but I hate bugging them at all hours for ice...I feel like a moocher.

1st of all you're paying waaaay too much for ice. I pay $5.50 for a 186 quart igloo full (a little more than two totes). If you don't get a machine you need a new supplier. :) I would not put a restaurant type of machine on a boat, the constant humidity and occasional salt water will do in the icemaker. I've seen it happen at marinas. If you really want to get one, keep it at home in the garage or someplace out of the weather.

A 10 or 12# bag cost $5 out here. This ain't NewBedford where you can pull up and have a hopper discharge it and they charge you by the ton. Ice like everything else is expensive here.

Like I said, I can get it for free but I hate bugging my friends and hauling it around and storing it.
I don't plan to run seawater thru it just freshwater I might be able to mount it else where and have a hopper or hose gravity feed it into the fishbox but I worry about clogging . It would only be on when at the dock and I need it. I think I can build a container that houses it and keeps it dry.

I would love to have a setup like this. A few liveaboards at my place have small icemakers on board they will make about a half bucket a day but if it was allowed to run 24/7 it would be the nuts.
I get ice on the fish pier the place I sell fish at hooks me up. Theres another place on the pier that gets 7 bux a tote.
The shaved ice packs down better and keeps longer.
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as you know salt water kills all, and I bet it would love to get its corrosion on an ice maker, which will = rust which = stain. get a 2 wheeler dolly and install an ice maker at your home and hump them to the boat (then sell 10lb bags on the black market for $3). Look at it as a workout, not a chore :)

get a 2 wheeler dolly and install an ice maker at your home and hump them to the boat

A good friend of mine did this last year...Hand trucks to the boat every trip. Picked up a used water cooled machine and keeps it in his garage, the cooling water trickle waters his garden. Makes up to 400#/day and is pretty quiet.
I can do it as well I just don't want to become an ice mover. I want to expend my energy reeling in abnormally large fish and digesting great food and also for bedroom activities.:) (in that order!!)

I almost bought a comm ice machine last year, maybe this winter I will pull the trigger. I still keep thinking I can install the head on-board and keep it somewhat free of salt...but I hear what you are saying, some of the components on these things are light duty steel and will rot out damn quick if exposed to a lot of salt air. But OH! whould it be nice to come down to the boat in the morning, open the fishbox and see 300# of fresh cubes in there!?!

These sea-ice machines start at $10K