Hypertext Markup Language (HTML) is the standard markup language for creating web pages and web applications. This step is important to keep your costs down and still get what you want. If you're not familiar with basic HTML and basic concepts about running a website, invest in the time to learn; it will be well worth the outlay in the long run. Even if the site is basically a template for you to use, you're still going to need to know how to insert images, create hyperlinks, and do some text formatting.
As an affiliate for Boatbookings, you will receive 20% of their revenue - effectively meaning your commission will be about 4% of any sales from your referrals.  You also receive 10% of any commissions Boatbookings make on repeat customers who were your referrals. They do have a minimum charter value of 3,000 ($/€/£/etc) before commissions are earned.