Hi @Charles,
Great to hear you had a lot of fun with your first ROV dive!
There’s a quite extensive post about both active and passive compensation of mass/buoyancy tuning here, which will hopefully be helpful to you:
“Slightly positive buoyancy” just means that the object will float back up if submerged in a liquid of the specified density. The speed of floating up depends a lot on the amount of buoyancy/density of liquid, along with the shape and size of the object.
For the BlueROV2 the technical details tab has a section at the top for physical properties, which specifies that a standard BlueROV2 has net buoyancy (without ballast) of 1.4kg, which means basically that when it’s submerged it has the equivalent of 1.4kg worth of gravitational force pointing back up to the surface. The weight without ballast is 9-10kg, so if you assume 10kg the acceleration upwards should be \frac{1.4kg \times 9.81m/s^2}{10kg} \approx 1.37m/s^2. If you assume no drag and a zero starting velocity then the time taken to go up 20cm should be about t = \sqrt{\frac{2\times 0.2m}{1.37m/s^2}} \approx 0.54s, but you should expect it to be longer than that since drag will slow it down. Note that that’s based on the water properties that the buoyancy was measured in, so is only a rough estimate, and is also only valid for the no ballast case. You can do similar calculations for different configurations or ROV designs if you want to.