I think that you also have to consider the work and expense involved with the jerry jugs. They are difficult to store, often rot in the UV, leak and become contaminated with salt water, lose the tops etc. not to mention the hard labor of hoisting them onto the boat, and then siphon, or pour and lose a couple of gallons etc. I have experienced all of the above, and that drives the per gallon price down alot. Then again, a trip to the dock to get water can become a whole day affair, and a social activity. always a trade off of what to do...
capacity should be figured by how many gallons a day you expect to use. 300 gal a day system does not make sense for a 40' boat.
A 300 gallon a day system actually makes great sense for a 40 footer. You make just over 20 gallons in 2 hours and only have to run the system every third day or so.
Biking water would be tough in a lot of places as the roads pretty much suck. You might be better off with a small cart with oversized wheels that you can pull along behind you. When carrying water for a distance I usually bring a walking stick and put it over my shoulders like a yoke with a jug on each end. I haven't had to walk water more than a few hundred yards as of yet. Its definitely no fun carrying jugs.
A lot places, even in the Bahamas, water can be hard to come by. Last year running low on water drove us out of the Raggeds after only a month. God forbid something happened to the RO system in Georgetown, there would be a lot of people SOL.
Considering the initial cost and upkeep I wouldn't have a watermaker if we didn't live on the boat. Even while living aboard I wouldn't have one if we didn't leave the states.