ORIGINAL: JanSorensen
As for adding movement cost. 2 MP is probably too much - but I do like the idea of adding 1 MP cost atleast. This will make surface raiders less able to roam far from their own bases - something I find a bit too easy currently.
You know, this could be another good application of oilers (in the hoped for GGWAW II). Allow naval vessels to fuel up only so much at a time, even within a given turn, so that exercising a naval unit's full movement would require refueling somewhere near the midpoint. This midpoint fueling could be made difficult if that midpoint is far from any friendly bases and in the middle of enemy waters.
There are a few ways to implement this, the trick would be to come up with the simplest way.
For example, say you divided supply consumption for naval movement into 3 types:
1) 1 supply to move the first zone
2) "short range fuel cost", can be loaded/consumed at any time, allows movement to 2/3 (round up to integer) of total range (I mean "speed").
3) "long range fuel cost", can only be loaded after the unit has already travelled 1/3 (round up to integer) of total range.
SRFC for LF=1, for CV=2, for HF=2.
LRFC for LF=1, for CV=1, for HF=2.
So the total costs for full movement would still be the same. This would also allow shorter movement to consume less supplies.
So, e.g. a HF could move 1 zone for 1 supply, or up to 7 for 3 supplies, but if it wants to move 10 it will consume 5 supplies and the last two supplies must be loaded after the HF has already consumed 4 movement points, which in this case limits its range into enemy waters without an oiler going with it.
If an oiler has a capacity of 5 and movement of 10 (that's about the same as transports ... it would take 6 oilers to move 30 supplies 10 movement points, and it takes about 5 transports to move 30 cargo over 5 double borders) it can support 2-5 naval units that are operating at max range but starting from adjacent to a supply stockpile. If the naval units remain close to the stockpile, the oiler could make multiple trips to support more units. But it would take more oilers to support naval units operating at maximum movement and never coming close to a land based stockpile for refueling.
It's not that this makes long distance operation impossible. It just gives it its own set of costs and tradeoffs, since you'd have to stock up on oilers which aren't good for all of the other stuff transports are good for.
Not perfectly worked out, but it seems like something along these lines could be made to fit in perfectly within the scope and scale of the existing game, and help fix some of the weakness of the naval model.