So, I ended up picking one of these up - I didn't purchase the one previously listed but rather this one (Twin Hornet 45) :
https://www.amazon.com/gp/product/B00F996452/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1Reasons for picking this one up instead of the other one :
1) Cheaper.
2) Dual Components (no single point of failure other than power / grid)
3) Waterproof (completely submersible)
4) More powerful (700w)
I put this in the bilge last night and all seemed well. I didn't wake up to a boat fire this morning so that was good! Prior to putting in the boat, I tested by placing in the freezer for an hour and then plugging it in to see it in action per the instructions. Everything seemed to work as it should (kicked on for a few minutes as it came up to temperature and then automatically turned off). This is a pretty robust little heater.
The biggest downside of this heater by far is the electrical cord. You can tell this is a company built by and run by engineers as this thing was made to be super reliable, simple and foolproof. You literally just supply power and that is it. No switches, dials, etc. for the user to screw up. As such, the electrical cord isn't detachable (one less point of failure) so this device would be pretty awkward to permanently mount in your boat (see pictures). The cord is so long that they must have been thinking about minimizing risk from bad extension cords or an extension cord that gets unplugged, etc. It is kind of over the top for my purposes.
I bought this anyway as I don't plan to permanently mount in my boat - just want to bring it out to extend the boating season / cheap insurance against cold snaps.
Overall - happy with this and I'll sleep better on cold nights when the boat is still on the water. I would not recommend using something like this in lieu of winterizing.
Hope this helps!