I am looking for a small-size horizontal current meter suitable for BlueROV2. If you have suitable recommendations, please contact me. My email address is wby@aohi-tec.com. 
I’ve used both of these:
Hi @Lucy, welcome to the forum 
Are you asking about electrical current-meters, or sensors for measuring water currents?
Thank you for your reply. I’m looking for sensors that are used to measure water currents.
Thank you for your recommendation. Maybe I didn’t make myself clear enough. I’m looking for sensors that are used to measure water currents.
I expected as much.
I don’t have experience with measuring water currents, so I’ve asked internally for any current meter suggestions our engineers might know about/have used
Following up on this,
Having looked into ADCPs a bit more, it seems like Nortek has some vehicle mounted options for ocean and coastal current measurements, although I expect that’s intended for measurements of horizontal current in a vertical column, whereas you may be asking for measurements in a horizontal row/area at the depth of the sensor, like from this horizontal profiler (intended for mounting on walls).
Teledyne also has a variety of ADCPs, although it’s unclear (from a brief look) if any of them are intended for vehicle mounting. There are likely other options from alternative companies, but it’s not something I’ve come across much so you’ll likely need to search for them