FYI, there are some issues with Unreal for naval simulations. The worst of which is the limited world size. For a single player game you could just use a streaming level, but you'll be responsible for all physics and simulation outside of what UE4 can do. You can also scale all your stuff down and pretend everything is 1:10 scale model.
It's also hard to model the curvature of the earth for your 3d rendering. Additionally, VaOcean, what I picked to do the ocean with, doesn't extend very far. It also has some gaps in the mesh when you try to make it huge.
I'd also caution against picking GPLv2 for your license then making everything closed source. You're responsible for distributing or making available the source code as soon as you release a binary. If you don't, that's a GPL violation and any of the contributors can sue you. I'm not a lawyer or GPL expert, but that's my understanding. It's also explicitly against the UE4 EULA to GPL a UE4 project, and I think Unity's license is GPL-incompatible.
If you want to make it closed source, immediately get in writing the consent of all contributors to relicense it and take down your public github repo (if you want). If you want to make it open source, immediately get in writing the consent of all contributors to switch it to a permissive open source license like MIT or BSD.
http://stackoverflow.com/questions/5...be-re-licensed
Best of luck!