OK, as has been suggested, start by practicing your fencing in the dungeons. There's one under Oxbay town (entrance just across from the shipyard, or dive off the pier and swim round to a cave). Another is in Redmond (entrance through an arched doorway near the church). There's a cave between Greenford and Oxbay. And there's a dungeon in Douwesen. You'll have to fight lots of skeletons and you'll have to learn how to fight two or more opponents at the same time, and if you survive long enough to find it, each of these dungeons contains a nice sword.
How to fight multiple opponents: keep your block up, wait for them all to strike at once, then hit back at one of them. If he's just hit you then he's not blocking, and if all of them have just hit you then there's a brief moment while they recover before they can hit again. Start the fight by shooting the enemy before you close to fight with swords - unless you have bought a bigger gun, you probably won't kill him but you will weaken him.
The initial deck fight should be you plus three crewmen against four enemies, unless you're <i>really</i> badly outnumbered. So you should start the fight facing only one enemy personally. When you've finished him, go to help your crew - the more of you survive the first deck, the more of you there will be if the fight continues to another deck.
And look for small enemies to board and capture before you get to the escort mission. By the time I'm "escorting" the barque, I'm in at least a xebec, possibly a schooner.