This is a new feature (added late 2012).
At coastal regions, you can embark ships and travel by sea. While at sea, your available actions and orders are dramatically reduced. In addition, sea travel always uses up provisions, since there is no land to forage.
Sea Travel works slightly differently from land travel.
First, you need to embark ships. This happens as soon as you press the button, and you are no longer considered inside the region as you are now at the coast, loading men and material into ships. Embarking has a substantial cost in both time and gold, depending on several factors including your unit and the location you use to embark.
The most important factors are:
- more men will be more expensive and take longer to load
- cavalry is very expensive and takes longer to load
- siege engines are very, very expensive and take forever to disassemble and store
- other Paraphernalia are fairly cheap
- a harbor in the region you embark in makes a huge difference, making everything both cheaper and faster
- being in a friendly (or own realm) region makes things cheaper
- retreating unto ships from an enemy region is the worst option, as it takes longer, costs more and you might lose some equipment and/or provisions
Once on ships, you need to reach the open ocean first. This will happen at the next turn. Then, you can choose to either land somewhere else (see below) or sail to other sea zones.
Sailing is much like land travel - you pick your target and every turn you will move closer to it. The main difference is that travel does not depend on your personal hours, because you can do all sorts of things (well, the few that you can do at sea) while the ship is moving.
So while sailing times might look crazy (20-30 hours are common), you will actually travel 12 hours every turn, 24 hours every day.
When you have reached the sea zone bordering your destination, you will land at the coast. This, again, takes your personal hours.
The most difficult and dangerous part of landing is when you land in enemy lands. You can not scout while at sea, so you do not know how many enemy troops are waiting for you and if they are guarding the coast or not. If the region is heavily defended, you will suffer considerable casualties even before you enter battle with the defending troops. Marine landings were largely unknown in the time period BM is set, and while unloading at the beach, your troops are easy targets. However, the more attacking troops land in the same turn, the fewer casualties each will suffer, as you can more easily suppress harrassment and ranged fire.
On the defending side, if you are afraid of your enemy attacking you by sea, the same rules apply just in reverse.
First, you can not scout sea zones as they are way too large. You can patrol them (to be implemented) so you get at least information about the number of ships sailing through and their flags.
Second, any defenders in the region that is being invaded, will cause massive casualties to landing armies. Especially if they have the support of fortifications. A well-defended fortress is virtually immune to assaults from sea.