It is worded as such:
Augment Summoning (SU) wrote:
At 3rd level, any creature with the water subtype the undine adept summons with either summon monster or summon nature's ally gains the benefits of the Augment Summoning feat.
This ability replaces trackless step.
Augment Summoning (feat) wrote:
Prerequisite: Spell Focus (conjuration).
Benefit: Each creature you conjure with any summon spell gains a +4 enhancement bonus to Strength and Constitution for the duration of the spell that summoned it.
Being that one is a supernatural ability and one is a feat, I would assume they would stack, although only for creatures with the water subtype.