Yeah, the naval system is a little wierd. If Sweden wanted to invade Britain it would take him 4 whole turns! One to build the units, another to embark the armies, another to move, another to unload and attack. And from the start it would be perfectly obvious to Britain and he'd have four turns to prepare...
What I think works well but also simply is this:
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An army can move ONE space on land, OR it can move indefinitely on sea - that's up to the fleet.
A fleet can move ONE space on sea, AND it can embark passengers up to a total of four from any 1 location, disembark any passengers into 1 location, and also pass armies FOR FREE to any neighboring fleets. And it can carry armies indefinitely of course. So a fleet could embark some, move, disembark, and act as part of a convoy [NOT the tail end of the convoy though, because it already spent the disembarking part of its move].
Also, it would help if armies could move when created, but NOT over a border when created, that works well.
So, in that case, Sweden would take 2 turns to invade. One to build the armies and fleets, and move the 2nd fleet to sea C [it could convoy the armies over now, except for the border-crossing rule], and one more to convoy them all over. Enough to take Britain off guard, but not surprise him completely. After all, he'll see the convoy coming...