Water does not get significantly more dense under pressure. According to Wikiped...

lmkg on April 7, 2011 | parent | context | favorite | on: Richard Branson Announces Virgin Oceanic Submarine...

Water does not get significantly more dense under pressure. According to Wikipedia, at an ocean depth of 4km, its volume is reduced by only 1.8%. That's not going to have to have much effect on the speed required to get lift.