Binder of the Sea (3.5e Feat)
From D&D Wiki
 Binder of the Sea [Binding]
Allows a Binder to bind creatures of the water to a greater extent.
Prerequisite: Knowledge (Planes) 6, Elemental Binding Class Feature
Benefit: Your Effective Binder Level is 2 levels higher when using Binder's Pact on creatures with the Water subtype. Also any summoned creatures with the Water Subtype gain +2HD but are still classed as their orginal HD for the purposes of Binder's Pact and the Type's HD Limit.
Special: Can be taken by a Binder as a Bonus Feat.