In their simplest form, a Jobs Board is the electronic equivalent of classified recruitment sections of newspapers, where people apply directly to companies that are advertising their own jobs. Same principal - different advertising medium.
A Jobs Board is not the same as an agency web site, which displays jobs that the agency is recruiting for, where you apply to the agency and not their client.
When using/signing up to any on-line recruitment site, whether it is an agency site or a Jobs Board, there are some simple rules you should follow, to protect yourself.
1) If there is a charge to register - don't do it. In most countries this is illegal, anyway
Sites that boast thousands of well paid jobs in the Offshore and Cruise ship industry and charge you a joining fee, are unlikely to have any jobs at all - this is just used as bait to draw people in. These same web sites will offer to e-mail your CV to thousands of companies as part of your joining fee. No company with any sense would sign up to a service that flooded them with CV's in this way. So, it is safe to assume that they have no clients either. However, if you fall for a scam of this type - in addition to your money, they will have your personal and credit card details. If they cannot get rich out of joining fees - identity theft is a lucrative alternative.
2) Make sure you know who you're dealing with - Look for the company's business address and contact details on their web site. This is a Data Protection requirement for all companies operating in the EU.
3) Avoid companies that operate their "business" via a free e-mail address.
4) Never pay an advance fee to an "agent" for medical examination fees/ travel costs to join a ship. This is another popular scam.
5) Don't post your CV on sites where they are on public display.
6) Don't respond to free adverts on web sites that allow companies to post jobs without authenticating them first.