Hi Dave and
I have a Mariner 3 and I used to use it extensively on the Thames before I upgraded. The Thames is a nice forgiving environment although can be busy on sunny weekends and you will need a license.
The Mariners are good solid boats for rivers and lakes, but as you may have discovered, can be fairly heavy to carry, especially if you have the slatted floor. My Mariner 3 has the inflatable floor so it's light enough (about 20kg) to carry on quite long walks to get to water. I did take mine out to sea with a 6hp engine a couple of times, but I really wouldn't recommend it. The flat floor and lack of proper transom are rather limiting.
I think the Mariners are a good way to get a feel for inflatables for a nice low price, but they do have their limitations. If you get the bug you will probably end up moving to something more sea worthy quite quickly.
Cheers
Simon