Only invasive strong growing aquatic plants can cope with such a depth, lol
Papyrus is a monster as far as clump forming growing habit is concerned.
It will go from a perty dinky plant to way too big and heavy to lift in one Summer season
If you can find a suitable tub, lotus will trot round and round within though how good that is at phyto filtration, no idea.

I don't think such a plant exists. All plants I know of that can be planted 30" underwater would not be tall, at all.

All of the tall plants would be called marginals. They do best with just wet feet. As they're planted deeper and deeper they do less and less well. In general they die below say 10-12". But the max depth varies with plants and conditions. For example growing a plant tall outside the pond and then putting it into 30" of water might not kill it for months. Water temp would probably be a factor too.

I generally don't put plants directly into a pond. I build a shallow "pond" or bog at the pond's edge instead and plant marginals there. Gives me more control, less crap in the pond, easier to work with the plants. When the plants grow they appear to be inside the pond. So that might be another option.

Depending upon how large your raised pool is, papyrus would work if planted in tall tubs, such as 5 gallon pots. The problem with papyrus is that it is invasive and will grow through the walls of the pot by the sheer mass of the roots breaking the pot after having breeched through the bottom drain holes. However the plant can be cut back as you wish.
As far as filtration qualities are concerned the best kind of plants for that would be surface plants with extensive root systems such as water hyacinth or watercress, both of which can be used in natural water reclamation systems. They, however, must be monitored for die back and all old decaying portions removed.

Here is a link that might be useful: info on papyrus

Max depths vary, but I've never read anything from growers saying deeper than 12" for papyrus.

Here is a link that might be useful: Papyrus

Some Typha were deep water plants, are they not?

albert_135, good find. I had to check and was surprised. Never seen cattails in natural ponds at that depth, but I guess I haven't seen every species ;). Of course being at the limit it might be a little tricky getting it going.

Found the perfect pot for papyrus. It is 20 inches high by 30 inches across. costs $27 USD. Can be found at
http://www.chulaorchids.com/html/black_pots.html

Cool site for pots. There'd be no lifting that pot out of the pond.

Lotus will grow at that depth and make an exceptional display.

Pickeral weed can go deep. Use a large tub and initially raise it up on some bricks until the plant takes hold. Eventually it will jump the pot and make a nice stand. You can get it in blue or white flowers.

Thalia is another plant you can sink deep once you get it started. Of course, water lily also.

Have you considered planting a tub of anacharis and letting it grow up to the top?

I've read that pink grapefruit water lily likes deep water. but the info I've found online is mixed at best. Watergarden.org says 12"-15" above crown while pondmegastore.com says 5"-48" from top of pot to water surface. I'm going to be trying some in a 4' deep section of my pond. I'm hoping it will work!

I'm trying cattails right now. I've had them about 12" below since November, waiting for the spring to see if they will grow.

Really would like to try the papyrus!

Sure, try papyrus, as long as you know what to expect, possibly a difficult clump to divide late Summer.
Umbrella palm and Cyperus isocladus are modest size similar to papyrus but more realistic size for a garden pond.
Thalia might be one to look at too, for a deeper water position