Subject: Have mammals that have adapted to the sea gone back to the land?

I'm curious to know if there is any evidence of land mammals that have 
descended from sea mammals (whales, seals, dolphins, otters).  I'm 
guessing the best way to know would be to check their DNA and see how 
closely related they are.  The reason I am curious is because 
evolution seems to be all about "filling available" niches.  When 
animals first left the water, the land had an incredible amount of 
environments with little or no competition.  However, by the time 
mammals returned to the sea, it would seem that the ocean life would 
have filled all available niches.  If mammals were able to enter such 
a competitive environment (the sea), would they not also be able to 
"re-enter" the very competitive environment of the land?  Is it 
inherently easier to go from land - sea?  Any dates of the history of 
sea mammal evolution would help.

Thanks 

Re: Have mammals that have adapted to the sea gone back to the land?