I do not know too much about the gps, but I have learned the hard way about waterproof.... I do a bit of sea kayaking on big lakes, and larger rivers (hudson around NYC)
You can get water resistant, waterproof and submersible.
If it is going to be on a kayak, you want Submersible, or IPx7 or IPx8,
Just waterproof is NOT, and will fail from water ingress while paddling.
I had a waterproof VHF crap out at a really bad time due to water while sea kayaking, and I was REALLY exposed (little kayak, big shipping channel, lots of large boat traffic), Now I get everything that is submersible at a minimum.
I had some good friends get into quite a bit of trouble in long boats, and I re-evaluated my safety kit, and now carry a bunch more stuff when I am in a long boat (PLB, 5 watt submersible VHF, signaling supplies, whistle)
The garmin GPSMAP64 is good
I am not sure about service, but a garmin/delorme inreach explore+ will store waypoints, give you sat messaging, maps, and an sos feature.
But it will require a subscription.
Depending on how far you wander, it may be a good idea to have a PLB instead of an inreach too if you ever get in way over your head.
Sea kayaking is a ton of fun but because the boats are fast it is pretty easy to get pretty far from home quick and things can get bad REALLY fast due to weather issues.