I don't have much to add to this discussion. I can confirm that Dire Charming Pendle Dashinger is the way to go. For some reason, whether it's because I levitated him or cast Night's Dark Terror on him, Pendle could not swim.
So, the first time, when I decided to play it safe and pull the Tangrin to the neighboring island on the east of Tangrin's island. The monkey fled at 19% health. He swam to the mainland in Field of Bone and Pendle couldn't keep up. Pendle swam like his swimming ability was about 1 and capped there. Strange, because he didn't seem to have this problem when I tashed him and pulled him to the neighboring island where I Dire Charmed him in the first place.
Round Two. This time, I tashed Pendle and pulled him to the northern end of the Tangrin's island. Since the monkey seems to swim well, and Pendle doesn't, I decided to give him as much room as possible to finish off the Tangrin before the ape made it to the water. No NDT this time, or levitation. Just good old Chanter haste. I parked Pendle in the shallow water on the northern end of the island and pulled Mr. Monkey.
Amazingly, Pendle owned this stupid ape. The Tangrin tried to run at 19% as he always does, so I got out my own melee weapon, Illusion: Werewolf and added what minimal DPS I could. The Tangrin made it to the water and I was afraid I'd lose him again, but to my amazement, he simply turned around and continued running away on his own island. He made it to the other side of the island and we killed him in the water.
No new information to add, except I can confirm that Pendle double hits for 177. The Tangrin CAN summon. I found this out during round one, when I became aware that according to what I had read here, this ape can swim, so I tried to pull him to the northern end of the island next to Tangrin's. As I called my DC mob off, the Tangrin summoned him back.
For some strange reason, after I looted the Tangrin and some Wizzie rune he dropped, I discovered that Pendle can swim just fine. As I ran back to the mainland, to turn in some bone chips for faction in Cabilis, I got attacked by some trash level skellies. Pendle attacked them like the dutiful DC mob he was. I was surprised to see him, since I figured that since he couldn't swim, he'd still be on the island. So, it wasn't the Dire Charm that somehow made him into a lousy swimmer. Must have been either the levitation or the NDT the first time.