Great advice, there is no grease whatsoever on the channels. I'm guessing this caused it to freeze up over time. I'll get a chance to pull the seats off this weekend and bring them home to see what can be done.After you remove the seats, soak the moving parts in PB Blaster for a day or so. The locking hardware tends to corrode and stick. If you try to muscle off the locking hardware, the parts may snap.
The actual part of the seat that slides runs on two strips of plastic. Hopefully they are intact. The channels they slide in tend to get gummed up with old grease. You can soak those too or try WD40 to soften the grease.
The base attaches to the underside of the seat with 4 bolts and washers. Revoving the base form the seat makes working on it a lot easier.
By the way, if the seat does not rotate right now, it probably will not lift up and off. You will have to try to get the pedestal locking rod to loosen. Do not try using a pliers on the knob, it will break. Try a vice grip on the shatf rod.
Appreciate your input on this seasick!