According to Steller "The manatees*)Steller confounded the seacow with the true manatee do not eat all seaweeds without distinction, but especially:"

No. Steller's Latin text from Miller's translation Probably the following brown and red algae*)Domning 1978, P. 112
1 Crispum Brassicae Sabaudicae cancellatum sea-cabbage with cancellate leaf Agarum gmelini, A. turneri, A. pertusum, Thalassiophyllum clathrus
2 Fucum clavae facie that which has the shape of a club Nereocystis luetkeana, Dumontia fucicola
3 Fucum scuticae antique Romanae facie that which has the shape of an ancient Roman shield Constantinea rosa marina
4 Fucum longissimum limbis ad nervum undulatis a very long seaweed with a wavy ruffle along the stalk Alarum esculenta

Design of the alga
Thalassiophyllum clathrus
They rejected uprooted kelp that drifted around.
"of the grass, which is washed around by the waves ... they do not eat.*)translated: Sven Waxell in Buechner 1891, S. 23"
They probably swallowed their food almost unchewed. Seacows are no ruminants, the food had to be broken down in their 150 meters long intestines. Their excrement resembled those of horses*)de Bestiis, translated by Miller, P. 32.
Their only food competitors were the sea urchins. These however were the main diet of the numerous sea otters, who kept the urchin population in check.
In the dark winter months the kelp growths almost halted, and the food supply became scarce. The seacows had to live on their blubber .
"In the winter the animals become so thin that, besides the bones of the spine, all the ribs show."*)de Bestiis by Miller, P. 43