It also depends a lot on the actual weight of the boat. If it is a commercially produced model then it should be available either on the boat or from their website. Also include the weight of outboard motor etc.
Cars will also state the maximum towing capacity in their manual. And as MOB said, it depends on whether you are launching and retrieving each time, or putting it in an out at the start/end of each season. If it is the latter, then you may be able to borrow a 4x4 for the job.
As a rule of thumb, the longer the wheelbase,the easier it is to tow something (I know, I have a two door Rav4 and a boat
- but I only two it about 11 miles).
And finally, four wheel drive is much better on a slick ramp, but if it is a necessity will depend on the gradient, and the state of the tide. For example at high tide you can launch no problem, but if you have to retrieve at low tide you have to drive over a lot of seaweed. As you slide back into the water is not the time to find out that your frontwheel drive fiesta is not up to the job
By the way, check out the driving licence requirements regarding towing a trailer. The new rules of the road specify the size and weight of trailers that are allowed on a B licence. Single axle and not more than 750kg if I remember correctly. After that you have to get a trailer licence.