I've been using cellular telemetry on my prototype. I'm using a Windows 10 SOC (have used RPI as well) with MavProxy which forwards to an amazon server also running MavProxy. From there, you can connect from any computer. It's actually easy to setup - after trying to find "a guy" for some time and not getting anywhere (at least with the windows system), I finally got frustrated and jumped in... then realized no programming is required!
Anyway, running a Windows system instead of RPI allows for you to put Skype on your boat and have a live HD video feed when you call it (and setup for auto-answer). You can also add Teamviewer to remote admin, or even put Mission Planner right on the boat, skip all the forwarding and such. Nerd-Redneck solution, I know, but it works and works very well. GIT R DUN... errr... warp speed.. or... uhhh.. nevermind.
Let me know if you need any help!