In merchant fleet industry employer pays ~$250 (~$100 for a rating and up to $600 for an officer) per month for every seamen crewing agency put onboard the ship, while this seaman is onboard. Thats $250 times the 2 million positions. That's $6 billion market.
In cruise fleet industry there are not just professional seafarer positions, but there are hospitality, retail and entertainment staff. The principle is the same to the merchant fleet but median is $150 per head. That's $150 times the 1.1 million positions.That's $2 billion market
Nobody knows how many there are people onboard private yachts. Supposedly, there are at least 2 million positions, but the market is not standardized and there are no decent researches. That's uncharted territory but it can't be lesser the $1 billion(?).