This is a use case that we are excited to see people use the vehicle for, exploring new places that have never before been seen by humans!
In answer to your questions.
1) The tether for the vehicle is neutrally buoyant in fresh water, so it will be slightly buoyant in salt water. This will have minimal effect on vehicle performance. If you look at the second half of this video it shows the third person view of the vehicle in the water and you can see how the tether handles. This was filmed in Hawaii in salt water.
2) The control device is kind of up to the users personal preference. As @Brendan mentioned a screen that works will in daylight is definitely a plus. Tablets normally have brighter screens than laptops, but we use both here at the lab. As we get more of these in the field and run user testing we will be able to give some better recommendations.
3) We have tested in a few high current situations. Since the vehicle is very hydrodynamic it handles going into the current well. Shooting video where you can see the performance is a little more challenging. The San Francisco area of the Pacific Ocean is where we test, but visibility is not great so shooting third person is hard. As a diver myself my opinion is that it would be able to handle any current that I as a diver would be comfortable getting into.
4) The video recording is onto whatever the control device is (laptop, tablet, phone, etc.). We are looking at tapping into the GPS information on phones and tablets to embed this GPS information into the video. More on this to come closer to the release date.
Let us know if you have additional questions, and I can't wait to see what you explore with the vehicle!