Who told you the ipe needed ventilation?
When working with it, especially machining , sawing, drilling, and sanding, breathing and skin protection is needed, especially if you have allergies or are prone to dermatitis or skin rashes. Most species classified as ipe are related to walnut and I hate working with black walnut. The natural chemicals in walnut and ipe that make it so durable make me itch like crazy when saw dust or chips get on my skin. And saw dust gets down the back of my shirt. But once I'm through making dust and hose my self down, everything is fine. Once you get it installed and sealed you should be OK.
You're gonna want plenty of ventilation any way. The moisture is gonna affect furniture, cloth, etc.
One caveat, I dunno what you are going to have in the pond, but those chemicals in the ipe will affect plant growth. In the wild they keep other plants, trees for example, from growing close to and competing with the walnut or ipe tree. Those chemicals are what keeps mold, algae, etc from growing on wood once it is cut. You'll want to seal all sides and cut ends before installing it to keep chemicals from washing into pond. They may affect fish, since ipe also resists termites, they will affect invertebrates like snails, shrimp, crawdads, lobsters, or what ever critters you got in there. Shouldn't affect crockigators, whales, walrus, or seals.
Except for Thompson's Water seal, don't use that, it won't withstand being that close to water.
I'd recommend several thin coats of polyurethane.
[CENTER][FONT=Comic Sans MS]Measure twice, cut once.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]Look at the nail, not the hammer. Watch the fence, not the blade.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.[/FONT][/CENTER]