Steller narrated, that the north-eastern littoral of Bering Island is covered with numerous rocks and rough cliffs, and during low tide becomes exposed for up to five verst [about 5 kilometres].
During high tide the seacows were grazing near the shore. When the tide ebbed they swam out to avoid getting stranded. Sven Waxell wrote*)translated from Buechner 1891, S. 23:
"It [the animal] never goes on the dry land, also never moves far away from the shore. At low tide it keeps away from shore, so that it will not strand. As soon however as the flood is back it comes again near shore and ingests its food."
And Steller:
"These animals are fond of shallow sandy places along the seashore, but they like especially to live around the mouths of rivers and creeks, for they love fresh running water."*)in de Bestiis, translation by Miller, P. 42
They probably drank the fresh water to "balance their electrolytes". One mayor question remains unanswered:
Was the Seacow able to dive,
or was it too buoyant?
Steller wrote on this*)in de Bestiis - translated by Miller:
"Half of the body, the back and sides, projects above the water
... it has to live, not in the depths of the sea, but it is always compelled in feeding to expose half of its body to the cold.".
No eyewitness ever reported that he had seen the animal fully submerged, even if pursued or harpooned. Sven Waxell reported*)translated from Buechner 1891, S. 24:
" ... its back is at all times visible above the water, because more sea grass [kelp] grows in the shallow water than in the deep".
If we assume that the animal was unable to submerge at all due to its low specific weight, this floating way of life would have had a number of benefits and disadvantages*)after Domning 1978, P. 130f:

The animal could graze comfortably the algae in the safe shallow water,
reduction of wave drag, reduction of heat loss,
sea gulls could remove parasites
Deeper plants are unavailable,
in subzero air temperatures problems of drying of the skin

It is hard to believe that evolution should have neglected these serious disadvantages.
Daryl Domning therefore considered*)1978, S. 130f
"The value of total submergence under adverse condition would probably have been too great for diving ability to have been totally lost; nonetheless the evidence indicates to me that for Hydrodamalis ... floating was the preferred posture for normal activity."