Do you know for sure how much your boat weigh?
Most family car will tow 1400+ some will tow up to 1600 might even be 1800 for a large estate, like volve. The site I use for car info is Parkers guild
Select the manufacturer, then model, you read all about the car, drivability, comfort etc etc. On the very last page, the model breaks down further to each trim level, and you can read about it's technical data like weigh and torque and horsepower. for example Vectra 2.0 SRi
It can tow 1200kg braked trailer.
Power Output: 134 bhp
Valves: 16
Torque: 188 Nm
Torque: 138 lb/ft
It has 134 bhp, this has little to do with towing, generally ignored (by me anyway).
Torque is 188Nm, Newton metre or 138 lb/foot. This is how much turning power, ie ability to carry the weigh/tow.
In a petrol engine car, peak torque is at high rev whereas in a diesel peak torque is way down the range, usually around 2000. For comparision, I had a passat 1.9 tdi, 115bhp, torque is 285Nm, I sometime has trouble pulling the boat + trailer (about 1200kg) up Lymington slip. To overcome this, I have to either rope it up or skew the car, so that I'm pulling up and across the slip and not straight up. Also, if you empty the boat first it weighs less, ie take unused petrol out, fishing tackles, icebox, food and drinks etc etc. I now have a 95 Toyota surf, it pulls without any problems.
I would be surprise if the Vectra will pull your boat out of Lymington as it's one of the steeper slipway I have encountered. Best way to find out is give it a go, do it when there are others about, if you have problems you can ask them for help.
Give me a shout if you want some help, I should be down this Sunday for the monthly comp.