First off, is this your dock? If so, you can construct whatever is necessary to make it easy. If is isn't your dock (as in at a marina) then you better check the marina rules as they generally prohibit fueling anywhere except at the gas dock.
I would expect the gas caddy to have wheels on it so you could roll it. Getting it out of the truck could be a challenge though.
How far from where you park the truck to the dock? Is it a gentle slope or are there steps? Would it be possible to dig a trench and bury a pipe to carry the gas down to the dock?
I am assuming that you need to gas the boat up at the dock everytime you use it, perhaps multiple times a day, so we are talking 40 to 50 gallons of gas a day?
If you have to carry the gas any distance then 5 gallon jugs are as big as you want to go. If the distance is short and you are a body builder type 6 gallon cans will work. I recommend the "Utility Jug" gas cans, they are much better at getting the gas in without spilling.
If you have a ramp and can use a dolly or other wheeled device then you can use bigger containers.
If you are using a lot of gas then I would consider installing a fueling system. Bury a pipe, have a gas nozzel on the dock and a large tank up at the parking area. Buy yourself a 100 gallon tank, fasten it to a trailer. When it gets empty hitch it up and head to the gas station, bring it back full and connect to the pipe that runs down to the dock. Get a siphon started and you won't even need a pump (assuming you have a slope down to the lake and you are not behind a levee).
Of course, if you have a lot of buddies that come to ride the best thing is to tell each of them to arrive with a five gallon can.......