For a 15-minute dive, the ASIWO Manta underwater scooter would be a suitable option. It offers incredible performance under water, and despite being the most budget-friendly option, it would be more than sufficient for a short dive of 15 minutes. It offers all speed settings, so you could choose your own speed, and it also has excellent thrust and efficiency in propulsion. However, it's worth mentioning that the riding time also depends on how you use the scooter as well as the depth and temperature of the water. You should always check the device and battery state before diving, and follow the manufacturer's guidelines and recommendations.