I am all at sea (ha ha) with 23 across "Most important sea?". I have ?A?N which assumes that 13 down is INAUGRAL and 14 down is ACETYLENE. I can't tink of the name of a sea that would fit and if the answer is "MAIN" I cannot see the connection with the clue other than that "main" could mean "most important". If it is "MAIN" where doe "sea" come in?
Any thoughts on this would be much appreciated.