Just about all my boats use plain belt drive and always have. Never had a problem with it whether 2mm or 4mm. No need to make the belt too tight, it just increases power consumption.
With a motor to shaft drive the tension can be adjusted by slightly moving the motor mounting. Between two shafts you can introduce a small adjustable tensioner pulley.
The whole point is to turn the drive pulley, not tug it against its bearing. Works OK on millions of car fan and accessory belts, works on models too!
Colin