Overheads on a boat are typically low and have minimal depth behind the headlining. Marine downlights are specifically design for this. Most modern marine downlights are now LED which has resulted in the form of the fixtures becoming more compact and smaller and surface mount fixtures can be used when there is either limited room behind the headlining or none.
Surface mount lights tend to be bulky so one consideration is head clearance in the areas you use them.
Older style downlights typically were halogen or incandescent and although these fixtures are still available to buy they tend to be for additions to existing vessels.