You should always change the gears manually if you are towing more than 800kg. You should also never tow more than 800kg in overdrive as it strains the gearbox. Leave the car in 3rd. You will do more revs but use the same amount of fuel as your engine won't be struggling.
I've towed a furniture trailer that was 1,050kg unladen full to the roof with furniture doing 110kph down the highway for 2 hours and my car loved it. Being a big square trailer with a lot of weight up high I did get a bit of wobbling happening when the wind picked up but you shouldn't have this with a boat. My only concern would be your car pulling that boat full of water up that boat ramp.
Hey. I don't understand why people are saying a boat will be heavier on the way out.
1 you have been driving it and will be less fuel. = lighter
2 you will have eaten and drink everything in your esky. Tip the ice out. = lighter
3 if your boat is filling up with water I would suggest you get it looked at.
As said. Just use the car as a manual and don't drive too fast. Always remember to check ramp before putting it in. Even if it is concrete.
Also change your diff and gearbox oil more frequently.
Just general maintenance too make sure your cooling system is on good nick etc etc
Your tow rating is based on whether you can stop it not whether you can pull it.
Tow bars are designed with huge safety factors, due to uncertainty about what they will experience. (eg people who snatch 4wds by the tow bar). With gentle acceleration in a straight line your tow bar will be fine with at least double what it's rated to.
As for towing - the most important thing is load it right. Around 25kgs of towball weight is ideal, no more than 50. New springs/shocks/suspension rubbers is probably a good idea - they are often shot by now anyway. A transmission cooler and new fluid probably wouldn't go astray if you are towing for any great distance. Be sympathetic to your auto and it should be fine if it's in otherwise good shape. Also, keep in mind that you can't stop, turn or accelerate anywhere near as well as when you aren't towing so keep large gaps from the car in front, be vigilant and try to drive as smoothly as possible.
Some upgraded brakes wouldn't go astray either.
I hear what ya mean by the water being in the boat
But mine never would take on a couple litres and thats a 88 model boat so i hope the new one wouldnt take on more LOL