Sanders are great and will speed things up but make sure to keep it on flat surfaces and lightly hand sand any edges or dont worry about the edges. I have sanded throught so much clear and paint when I first started. I just get close to the edge with the sanding block and buff to the edge. As for the grooves, if you go to TCPglobal you will need to look up the sanding blocks. There are some foam ones that have different shapes, I need to get a set myself since it will help you reach those odd contures in the boat and still keep a nice even sanding on it. As for sanding in a straight line you can go that way or in circles. If in a straight line make sure you sand in two different directions, I always just switch hands and it will give you two directions naturally.
If you can get to an auto paint store, ask them for an eraser and the stem to go with it. Use that on your drill and that will help take away the vynal graphics on the boat really quick, just don't push down to hard on it. Let the eraser work on it's own and apply a little pressure. You will get the feel for it and start taking off graphics really quick. I did one of my jet ski's last summer and it knocked out the 13yo vynal really fast compared to when I started by hand.