I would say it depends on the brand. I have LEDs made by AquaRay/TMC, and they are sealed. Hard to explain exactly, but there are two levels of sealing if you will. There is the casing itself, then a separate chamber sealed off from the first that houses the actual LEDs and circuitry. I’ve had them 7 years, and there is a ton of salt creep though the first chambers, but the inner chambers remains completely free from any salt or anything else. It’s a little off-putting seeing that much creep in there, especially when I can’t get to it without destroying the light.
This is just my experience with my particular lights. I’m sure advice varies widely depending on the manufacturer. Best bet would be to seek someone who has your exact brand and could give you a more pointed answer.