Just a quick comment on singledivers.com...
I travelled with them once and found the trip leader as disorganized as their website, Others May have had better experiences so if you try them and have a great trip that’s great!
I too am a single diver and I’ve booked several trips through some of my local dive shops that organize triips. Each time they were able to find me a roommate to avoid the dreaded single supplement!
One other great website is diveandtraveladventures.com
I’ve traveled with them several times and I’m already booked for 2018;as well as 2019. I’d highly recommend checking that site out. Heather (company owner and trip leader) will do her best to find you a roommate.