Your water table is very high it seems...You can either install it as a
raised pond above ground or install a French Drain under it to funnel the
water away from it..(doesn't seem large enough to install a sump pump under
it)...Did you encounter a lot of clay? Water while digging? Is it on a
slope? This is like putting your pond in a big bowl, the ground water can't
seep out fast enough...Like a ship floats...As long as it displaces more
water than it weighs, it will float..I've seen much larger ponds
float......What a sight (and mess)....Ever seen a gas station with big
bulges in the parking lot? The tanks are trying to float from ground
water....