Thanks to all for the help! I ran the encounter Tuesday night, and had a blast.I used KM's phased encounter (with some changes) as my template. The PC's were hunting the eel in order to obtain it's caustic digestive juices. So it spat acid, not electricity. I also gave it the ability to swallow anyone it grabbed with its bite.
The initial search for the creature was somewhat lackluster. We haven't played for a while, and everybody's energy level was a little low. After some arsing around, the players decided to try and lure the eel to them by chumming the waters near its hunting ground. I think mostly because they like to say the word, "chum". It worked.
The eel immediately knocked half the party off the boat and swallowed the ranger. I made a spot call that the ranger could make attack the beast from inside, which in retrospect was probably too generous. Also, the paladin activated his Wavestrider Boots and took the fight to the water. The cleric got out of the water and started working the ballista -- it critted. Phase one ended quickly.
After losing a quarter of its hit points, the eel wrapped itself around the PC's boat and started squeezing. The players were worried at first, but the wizard was able to hit the monster with a Ray of Enfeeblement, and its boat-killing powers were greatly diminished. The eel's attack rolls were dreadful. He only hit twice with his bite, and he failed to swallow his target (the paladin) the second time. Frustrated, the eel flung the paladin into the wizard, knocking both of them down. The ranger made himself into a kind of internal cuisinart. Before long, the eel was down to his last 60 hit points.
He turned to run, but the PC's were able to get their boat moving after him in time. The ballista critted again, and the mighty eel went down for the count.
If I had it to do over again, I would limit the ranger's ability to attack from inside the monster. He took a ton of damage from being inside the monster's gullet, but he gave back worse than that.
Anyway, a good time had by all. Thanks again for the help!
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