It's an interesting thought experiment, and I think undeserving of downvotes. If you could get a bunch of balloons underneath the seaweed to float it above the

It's an interesting thought experiment, and I think undeserving of downvotes. If you could get a bunch of balloons underneath the seaweed to float it above the water, maybe it would have a hope of burning -- but the balloons would have to withstand fire. Difficult. With a heroic effort maybe you could sweep the stuff onto a beach where it could burn. You'd burn a lot of fuel doing it, though, and then a lot of seaweed, and you wouldn't have changed the conditions that led to its growth in the first place.