noun
BAD : The first diver reappeared covered in seaweeds.
GOOD : The first diver reappeared covered in seaweed.
◆◆◆
seaweed is an uncountable noun: 'Can you really eat seaweed?'
noun
BAD : The first diver reappeared covered in seaweeds.
GOOD : The first diver reappeared covered in seaweed.
◆◆◆
seaweed is an uncountable noun: 'Can you really eat seaweed?'
Longman Common Errors English vocabulary. Английский словарь распространенных ошибок Longman. 2012