Brushed Copper Wall Light - GU10 Lights Up and Down

Is this light fitting suitable for coastal areas?
Yes.

Can the lights be used internally too, specifically in a bathroom?
These lights can be used internally, and having a high IP rating they are perfect for the bathroom.

Can this unit be used with just the down light bulb installed?
Err yes if that`s what you wish. The lamp wiring is independent.

The electrician has fitted the lamp, but did not keep instructions on how to change the lamps - unable to screw top or bottom open - how do you get the bulbs out?
Both top and bottom cyclinders have a standard clockwise thread. Put on rubber gloves to help your grip and start with top cyclinder. Grasp at top and turn anticlockwise. If you electrician didn`t use the oil supplied with the fitting it will be a little stiff but tapping with something blunt may help.

Is this product dimmable? Will it work with both a 250w switch or a 400w switch?
This fitting uses mains voltage halogen reflector lamps (GU10) which are dimmable. Total wattage is 70W so you would be able to control 3 of these on a 250W dimmer module (210W total) or 5 on a 400W module (350W total) assuming nothing else is being operated by the same dimmer.