You may not need contractor grade, but you need a pretty good laser level, at least in the $40 to $60 range. The two main points are that it needs to be bright enough and there needs to be a way to make sure the level itself is level! It needs to be plenty bright because the boat is curved and you have to back away a bit to light it end-to-end. A problem in tight spaces. And it needs to be fairly dark in the garage or shop unless you have a very bright laser.
I believe the best inexpensive lasers are self-leveling, i.e., they rely on gravity to come to level. The others rely on you leveling them using a bubble, which could be mis-calibrated and it's hard to get the bubble exactly centered. It the level is the tiniest bit off, it can be off by inches at the ends of the boat.
As to the need for a water line, my knowledge is much more limited, but do you absolutely need one? If you build the boat properly, she'll float "true to her lines" even if they are imaginary. Others will be able to refute me on this I'm sure. Or maybe you could "float your boat" in absolutely calm water and use a wax pencil to make several marks at the waterline. Paint the line later when the boat is out of the water and dry. Probably not a very easy thing to do on a trailer, though.
The "bottom line" it seems to me is that it is better to have no water line than to have one that is off!