Rich,
They have no problem living together but the Hyacinth would like full sun and the Lettuce would like part sun. Hyacinth if crowded will grow tall where Lettuce can't and if they get too crowded the newer plants will stay smaller. Hyacinth like a lot of food and will die off if not enough food is available but the Lettuce will usually just not grow as fast if enough food isn't available.
Mike

I have both in my pond, but the lettuce don't seem as green as before, kinda yellowish to light green. Will full sun cause the lettuce to get burned (yellowing)?

Thanks,

???In years past, lettuce has done well, beginning last summer it seems to have just about disappeared in this area - commericial and private and well -- just slowly turns yellow and dies. I may have 1 very small, very yellow plant left?????
Hyacinths doing great! Two years ago, we had them probably 3' tall in our bog pond.

Water lettuce that turns yellowish is probably suffering from coldness. One way that might alleviate the impact of colder weather is to be sure the plant is growing either in very shallow water (one inch or less) or on mud.

Here is a link that might be useful: Pistia

I guess nobody understood what I said. The water lettuce is doing great, the hyacinth is gone, mostly. always in full sun after about noon.

Or maybe it isn't water lettuce. I don't know, but it is a pretty bright lime green. I like it, what ever it is.

Rich