I have just been invited to help out with SEO'ing - Search Engine Optimizing Rebreather World. So before I reply to the invite, here is a freebe.
My underwater experience is rather limited. My first Scuba dive was in Vanuatu this year. Went on a cruise ship for 10 days and snorkled around the amazing reefs in Fiji and Vanuatu. It is an amazing world under there.
What search terms does RebreatherWorld want to rank for?

Really basic question, but it has an equally basic and profound answer.

So if you want to rank on Google for "Rebreather" how about linking to it with the text link Rebreather World - for the best in rebreathing, not Rebreather World. You guys are already top of Google for Rebreather World.

Then there are image links. Have the alt text as "Rebreather". Thats with Rebreather

Variety is needed as well though, so certainly keep some of the rebreather world links that you have. If/when you guys decide to hire me, there are bound to be a few more phrases that you can promote for the main root domain, plus some specific links for inner pages.

What about other search terms

How about links to specific inner pages that you particularly like. Link using the first words of the title of that page. This gets that page to the top for its search term. Better 1000 pages at the top of Google than just one page.

Google PR - High PR links needed

Now who has the Google toolbar installed? I use firefox, and have the PRGoogleBar extension installed. I can see that the main homepage of the site has a Google PR of a very middle of the road PR3. So as well as lots of links to get the link text value as votes for the site, you need links that are on high PR pages with not too many other links on them. The Google PR that comes in, drives not just that page, but every other page to the top of Google.

Now the visible PR updates (visible on the toolbar) only every three months, so there is a bit of patience required. Google has a live moment by moment count of the real Google PR, so it all counts.

Lots more things that have to happen on the website

With SEO, everything has to be done right all at the same time. So there are lots of steps that the website has to do - they will make the difference and get your website ranked high not just for its home page, but for each page on the site.

All the best.