If you use Follow Me and RTH, the drone will return to wherever your boat is at the end of the shot, since it knows your phone's GPS position all the time. Of course this won't help because your subject is a different boat.
The solution is to do a Follow Me on the boat you want and when the shot is over, manually retrieve the drone to your position. Since retrieving the drone safely to a moving boat is somewhat tricky, depending on the size of the clear deck available, I would suggest experimenting with two cars in a really big field. If one is not available you can use two people. Drive or walk in a loose formation after you set up the Follow Me on the other car or person. Move around in as big a circle or oblong as possible, and when the shot is finished, continue to move and bring the drone back to you and hover it 5 feet away while still moving. Then bring it to within a foot of your hand while still moving. Once you can do this you can try it with two boats.
This is NOT easy!!! Bringing the drone into within grabbing distance on a pitching boat moving even 5 knots is a real skill that needs a lot of practice. Forget about landing it on a boat unless it is the size of a cabin cruiser with a nice large obstacle free after-deck! You will have to grab it. If you can stop your boat it's a lot easier.
Here is a thread from back in August about this:
Launching from a moving platform (boat)
You will also have to practice the different Follow Me modes until you understand exactly what sort of shot each one does, as well as lots of practice flying precisely while you are moving. It's a lot harder to fly precisely at slow speeds than zooming all over the place.
And read this:
Advice for new pilots
There are hundreds of good videos on the internet. Just ask about anything specific, one of us is likely to know where to find out about it.
