50% because in the worst case you could still return back to where you came from. If there's an enemy in the way you'll have to sacrifice a part of your fleet to fight your way to freedom. For the last point I guess you can't really hang around enemy cities without a nearby supply base and even then you are always at a disadvantage unless yoru already have established naval supremacy, that is wiped out most of the enemy's ships.
A bit unrelated, transport ships still automatically skip their turn and are unselected when you right click on a land tile while having them selected, but not before moving next to it and unloading their cargo. I will remind you that it wasn't this way in BtS or even Vanilla RFC, there you still had the transport ship selected and it just carried on as if nothing happened after unloading its cargo.
Also a bit unrelated, naval invasions are too easy, or rather defending against them is too hard unless you have a mindboggingly huge number of ships, enough to surround all of your cities. Granted this doesn't really matter in practice because the AI never tries to launch a naval invasion in the first place, but that is not really good either. I think we already had a discussion about that in this thread sometime, but it never amounted to anything apparently. I have two suggestions:
1.: Modifying the Sea Patrol mission so that a ship that is running it automatically attacks any enemy ship that a) enters its range, b) unloads cargo or c) enters a tile next to a city of the ship's civ.
2.: Increasing the movement cost it takes to enter coast a whole lot while leaving ocean unchanged, so a Transport would take two turns from Hamburg to London but also from Ponta Delgada to Halifax. Or maybe just entering a tile that borders an enemy cities takes up all remaining movement, so your fleet is sitting ducks for a turn unless you already removed the enemy's defenses with air bombardment or manage to conquer the city anyway.