If life began in the sea how come it is possible to drown? is this a paradox or an unfortunate coincidence?

There is no paradox and there is no coincidence. We evolved right out of our gills and grew lungs and arms and legs. We do sometimes get in touch with our past by donning black rubber suits and masquerading as crippled seals for great white sharks, but that is about it. We are land dwelling air breathers now unless we reverse the process over the next couple of million years.