Backing any trailer straight without any steering correction is not possible. Even your car or truck will not stay completely straight without slight correction when going forward. The mistakes I see people make are: 1) They feel they must "twist" away aimlessly at the wheel in order to put the trailer where it needs to go. 2) waiting too long to make a correction, 3) making to much of a correction which then requires a counter-correction, and 4) looking out the side window, looking out the open door, looking in the inside mirror and then the outside mirror. Doing all of those in rapid succession is a good way to confuse and frustrate yourself.
Hands at the bottom of the wheel to get the right-left issue in your head is good for learning but once you make up your mind the trailer is going where YOU want it to, not where IT wants to go you will quickly drop this technique and just steer the darned vehicle. You know how wide your trailer is so when you plan your attack, look at the area behind you before getting in the vehicle. Make the drivers side of the trailer and tow vehicle your guide. Pick a spot on the ramp that if you keep the boat/trailer lined up on you don't have to worry about the other side. Yes -- you need to make sure nobody violates you space while backing but that's easy and what mirrors are for. Not so much of a problem with a recreational boat but with a travel or cargo trailer a rear vision camera would be handy.
I have a cargo trailer that gets backed into the third stall of my garage. My challenge is to get it in the garage after stopping on the street, without having to pull forward. I'm about 80 - 85% successful. Again, I know how wide my trailer is. I therefore know that if I miss the door by 6 inches or less on the drivers side I have plenty of clearance on the other side. On the garage floor I have a strip of black/yellow caution tape that when the left side trailer wheel is on that line all I need to do is back up until I hit the stop block. The other trick is to always back up from the drivers side. Backing from your blind side can be more of an issue but again, practice and use of mirrors is essential for good backing technique.