Yes you can as long of course within the amp rating of the switch. A standard light switch is rated around 6A which for 230v is about 1.4kw but with 12 volt that is only 72W.
Also most spot lights are rated at max 12 volt unless designed for vehicles then designed at up to 14.8 volt.
With a boat because the engine is not running all the time (Unlike a car) the charging is often what is referred to as step charging.
This means when the engine first starts it charges at max until around 14.8v then maintains that until the current drops to around 10A and then drops voltage to 13.2 volts.
As a result what you select must be able to take the peak voltage or have some form of volt dropped.
LED's are mainly worried about current not volts and each has a voltage of around 1.2v so in theory 10 at 12 volt however they can easy get thermal run away so more likely 8 and a resistor. The less lamps and bigger the resistor the more able to take over voltage.
The standard florescent lamps used in caravans and boats turn the dc into ac then transform it up to around 70vac at very high Hz and the electronics also compensate for voltage variations.
LEDs can have a certain voltage tolerance, but the best for them is to keep both at a stable rate. As far as I know, LEDs can be manufactured as constant voltage or constance current, so depending on which type you purchase you have to worry about over-voltage or over-current. My LEDs are all constant voltage, and if you try a search in Google for these terms, more results come up for constant voltage LEDs. If the charging of your battery is not regulated, than use this converter: Google "ledcentre.uk.com/272-led-light-accessories/325-dc-to-dc-12v-converter-5a.html" or anything similar to this, otherwise the voltage fluctuation might ruin your LEDs in a short time.
Take care! although as Eric says 6A or even 12A rated switches are for AC where the power goes through zero 100 times per second, so switch only has to break load for a millisecond; not so with DC which will arc and burn contacts unless very quick break action switches are used.
Even overrated 20A AC switches will burn on a very light DC load.
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