Questions like yours are why I said to go to a drone specific forum, you'll get much better answers.
Don't ever take off under a tree, or any other obstruction, be sure to have a clear area, and around the area, as the RTH function needs some fudge factor.
I don't have a P3, so I can't tell you specifics, but you can test your questions easily, but be ready to go into manual mode if you need to take over any automatic functions. Because of this, it's very important to do a lot of practice to be capable of flying in a manual mode, in case it ever screws up and starts doing something you don't want it to when in an auto mode.
Yes, flying from a boat has many unique problems to think about! RTH is one of them. Even if you set the home location when you are anchored, what if the boat drifted on the anchor? At least it will come close to where you are. Also, if things go wrong, it will end up in the water, at least on Winni that is fresh water, but drones don't float! Also, those propellers can cause damage to people, isinglass and perhaps canvas, plan and be safe!
Follow me mode hopefully will do that (follow you), but what if something goes wrong, it probably will go to your home location, but if a drone was designed properly, when in follow me mode, you may want the drone to come back to a place where you were recently, OR you may want it to go home (every flight could be different). You also may want it to do a "hover and wait" on RTH, so if you have to get your boat into position before it lands in the water before you land manually.
I've been flying a drone for a while, but not a P3. Good luck!