You gain a Swim speed of 10 feet, plus +5 feet per Tier of the armor. If you already had a Swim speed, you instead gain a +5 foot Bonus to that speed per Tier of the armor.
You gain Echolocation as an Imprecise Sense while underwater, with a range of 15 feet per Tier of the armor.
You can breathe underwater as if it were air, take no penalties to attack rolls, saving throws, or skill checks due to being underwater, and do not take any damage or suffer any ill effects from the high pressure of deep water environments or from changing depths too fast (i.e. no worrying about the bends).