With all the gear and Charisma that comes from a PC, the sanguine (butchered spelling) will know when and when not to board, on what side, as well as who is the greatest threat. When raiding, they'll watch the ship and plan it out first. With their high Int, this isn't much of a stretch.
Wait until a PC isn't on watch. If the PC's rotate shifts, attack from a side they're not on. If all the PC's work at once, even better! I doubt they'll stay up all night...
. Unless campaign rules differ, an NPC shouldn't have a Spot check of more than +8. Most will be much lower.
Standing guard is tediuos work if there haven't been any recent encounters. Have the guards roll endurance checks to not fall asleep.
Sanguine have claws I think? If not, they can make climbing claws. Ropes won't be needed, especially if the ship isn't de-barnicled (I forget the name of the process...) regularly.
If the ship is carrying cargo, it'll lay low in the water. Have the deep sea critters make a hole wide enough for them to slip through. Even if noticed, there isn't much the pc's can do until the hole is made. After that, the ship rapidly fills with water, giving the beasties home court advantage. PC's being what they are, they can (hopefully) figure out how to repair the ship after the battle.
Make it guerilla style tactics. Don't make a single raid; hae the sanguine take one NPC a night, either via sniping (they have those damned speargun-ish crossbows) or by having a rogue climg up and take out a look-out at night. Leave the body to be found. Paniicy people make mistakes. Have some of the crew start going nuts, maybe killing each other at the extreme. When numbers and morale are whittled down, make the mass raid.