The Mcurdo website has a comparison document, you can get an add on for the 210 if it is to be used near water...
http://www.mcmurdo.co.uk/doc/GPS%20PLB%20Comparison.pdf
(at the bottom)
It reads that the buoyancy is there to ensure you can retrieve it if dropped in water, nothing about self righting and operation whilst in water.
Reading the manual from the website, it states...
Using the PLB afloat
The PLB is totally waterproof but is not designed to operate when floating on the surface of the water. It is buoyant and will not sink if dropped into water, if possible keep it out of the water when in use, place it high up on your PFD, lifejacket. or chest and clear of any wave wash. Any trapped water should be removed periodically by shaking the PLB.
So I guess the biggest technical difference between the 210 and the MaxFind is 24 vs 48 hour operation and user replaceable battery ?