Cost per mille requires only that the publisher make the advertising available on his or her website and display it to the page visitors in order to receive a commission. Pay per click requires one additional step in the conversion process to generate revenue for the publisher: A visitor must not only be made aware of the advertisement but must also click on the advertisement to visit the advertiser's website.
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.