Is Ethernet PoE direct burial safe in a submerged implementation?

Welcome to SuperUser :-)

Check out Wikipedia's PoE-page. An exposed wire will give a jolt, but those are hardly fatal values. The cable category affects speeds, not the amount of PoE power.

You will obviously need to protect the wire. You should choose cable that's intended for underwater usage. Belden has a nice article discussing direct burial cable. I would still run it through a flex tube, it doesn't add much to the cost or make the installation more difficult, but does add another protective layer. The cable reseller can best advise on this.

Another option would be creating a wireless bridge between the pole and the shoreline, but you'd still need to pull power to the pole, so I wouldn't bother.