A very interesting question. I think to start we have to look at how an inhaled poison would work in the natural environment. The particles of the poison diffuse through the air until they are inhaled by the unfortunate victim. For this to occur two things have to happen.
- The poison has to diffuse through the air.
- The diffused poison has to be inhaled.
Now if we look at what happens with water breathing and any sort of "diffusion" through water. With breathing water the character can breath the oxygen in the water, similar to how any fish does, except magically. So for them to be poisoned underwater it's realistic to expect a poison to be in the water they are breathing.
Whether an inhaled poison could mix with the water or not, I am unsure of this, but think it unlikely. And as you mentioned a contact poison would need to be very concentrated to make it work in a large body of water. However it is possible that you could use an ingested poison in a very similar way in water to an inhaled poison in air.
Whilst this is definitely unofficial, if this were to come up in my game I would rule that ingested poisons diffuse through water like inhaled poisons diffuse through air, and as such any player who swims through, and breaths in, the poison is subject to it's effects. I would also rule that because it is being breathed in, rather than ingested, the time for it to take effect is reduced to instant, or mere rounds (as per your need), as it enters the blood stream much faster.
To be fair, you could limit the poisons that could be used to powders and liquids, as they are the only things that could diffuse through water.
tldr; You could say that ingested poisons work in water similarly to the way inhaled poisons work on land.