Get the Mavic! I brought my Phantom 3 4K from US to South East Asia, travel and lay over with a big drone in the bag is not fun, you get discriminated at the airport security. The Karma is still too big so you would have the same problem that you need a dedicated bag for it. The Mavic fold up small enough just like a power bank. And remember, the Mavic is a better drone. I don't even know how the Karma camera work, like do you set the goPro to start recording then take off the drone? If you can set the goPro to start record in mid air, what technology it uses for the remote to talk to the goPro? If it's Wifi, then the range is too short.
If you want video for boat ride or for land use, get the gimbal for your phone (around $300 for DJI Osmo mobile), the camera on the phone is just as good as a goPro and camera on the phone lets you do more, like timelapse/hyperlapse, white balance etc. Camera on the phone do not have fisheye problem as well.
If you still want to bring something small into the water, get a Xiaomi Yi 4K action cam, it's much cheaper than a goPro.