I use jerry cans to fill up before I set off, but when out and about I just use whatever is available at the marina and pay for the convenience.
So for me I balance convenience with cost depending where I am.
But lets not forget, we are not commuting to work in our boats, we are out having fun a few times a year. So for that I don't mind paying extra.
I can imagine its a H&S / Environmental nightmare to sell fuel in a marine environment. Plus they will be paying others to rent the spot, store the fuel on dry land perhaps etc. etc. So without the support / back up and economies of scale of a large chain I'm not too surprised at the price.
As we know, most garages make their money on the food and sweets, not the fuel so much.
Perhaps the marine chaps should start to sell us sweets