Has anyone ever used a residential hot water heater (elec) for a hydronic baseboard heat.

I'm looking to heat up a 600 sf apartment and was thinking of using this.

This idea has been used a lot but I do believe it is NOT legal. A water heater is not designed for continuous heating that it will be doing and the temperature of 150 degrees (minimum), will cost a small fortune. Your best solution is contact landlord about poor heating or get one of the infrared heaters that are on sale everywhere right now. They are much cheaper than the set up you want to do and pretty maintenance free. Good Luck

Some water heaters are listed for both space & domestic use but most are not.

You don't want to use an ordinary water heater for space heating because the constant inflow of fresh water will soon destroy the hot water baseboard units.
The baseboard will not withstand the pressure of domestic water & could quickly rupture.