The Seacat website has a security issue and this is maybe why it doesn't open for you

In any case, if you manage to fill in any dates it will divert you to the Buquebus website.

Seacat was a vehicle that Buquebus used to sell discounted tickets - but they have used the Buquebus vessels for quite a few years now.

I don't think they are actually using the site at the moment.

In any case they would be selling you tickets that you can already see on the Buquebus site