Can I use a smartwatch as a dive computer?

Only if it is an actual dive computer that also happens to be a smartwatch — Garmin Descent line, Suunto Ocean, Apple Watch Ultra with the Oceanic+ app for its rated range. A standard fitness watch with a "depth" sensor is not running a decompression model and has no business being your primary instrument. Purpose-built units also survive being purpose-built: buttons that work with gloves, screens readable at depth.