Hmm, AFAICT
The Kubota part 15321-73015 as in the L245 parts catalog (see Messicks). That is a 180 degree thermostat and is what came with the tractor from the factory
The Kubota part 19434-73014 (and -73010 and -73013 it seems), which is also Universal part 299798, is the 160 degree unit and is what came with the boat from the factory
I've read, but don't quite believe, the lower temperature was chosen to avoid salt precipitation in the heat exchanger. The sea-water temp in the exchanger doesn't really get that hot, as long as its working ok. I've also read that sea-water cooled engines can get salt precipitation issues which is more plausible since there are real hot-spots inside the block where water can get past the atmospheric boiling point and is why the system is pressurized. Of course, if I take a pot of ocean water and boil it on the stove I don't get salt coming out unless I boil it almost all the way down, so there's that. Maybe the marine diesel makers just always bought 160 degree units since they had salt and fresh water cooled engines?
A bunch of folks use the 180, especially fresh water cooled folks. I'm on the fence, and am days away from replacing my own with the 160 replacement I got. I'm stopping at the Kubota dealer this week to hopefully borrow the special 43mm socket to get the crankshaft nut off. I'll ask them their opinion. People seem to say the 180 gives a better running, cleaner, more efficient engine which is also what theory says, so I could easily be swayed. Plus I could probably take a little longer shower with the slightly hotter tank
Note also that super cheap thermostat deals often don't include the gasket which ought to be replaced when you have the housing apart. Make sure you're comparing apples to thermostats.