With the units you have, you'll need the King as General, so you can use eels as Battleline.
So:
- King
- Tidecaster
- 12-15 eels (6 and 6, or 9 morsarr 3 ishlaen)
- 2x 10 Thralls
- Fill with an hero of choice, or an Allopex if you feel fancy.
That would be my approach, I wouldn't play the Turtle or Eidolon in 1500pt, thats 2000pt material for me.