Looking at statute and definitions, can you have a UUMV on a personal watercraft?

Good point/questions; due to the lack of definitions. IMHO, a PWC could be considered both a "boat" and a "motor-propelled vehicle", making a SJF charge of UUV A-OK.

You may find McLemore v. State, 669 S.W.2d 856, of some use.
I do agree as mentioned before the lack of clear definitions does not help. Closest definition of vehicle I can see is 30.01(3), and while its in the same Title (7), it starts off with in this chapter (30). However, that definition would seem to support your position.
I might tend to stay away from the use of the word watercraft, since there is a specific definition for that under 49.01(4). While it should be limited to that chapter as well, why add something else to the definition conundrum.
Also as pointed out by the poster above, while it is often called UUMV, in fact it is UUV, and if you start saying motor vehicle, someone might want to make you stick to the definition in 32.34(2).