Which locator should I choose?

U1 vs A1 vs D1

By default, we recommend the U1, as it is extremely simple to deploy: GPS G2 is 'plug and play' when using an antenna and a U1 locator!

The only caveats with the U1 are that battery charging needs to be taken care of, and that it can only be submerged for about 6 hours at a time (the clock sync between the U1 locator and topside can have drifted, and the depth sensor in the locator does not tolerate being submerged for longer). If these are not issues in your application, the U1 is for you!

The A1 is a good option in most other cases. There is no battery, and no limit on running time. However, since it is connected by a cable to the UGPS topside unit, and since depth information must be independently provided to the UGPS, it is typically more complicated to integrate. We have a guide specifically for integrating with a BlueROV2: 
https://waterlinked.github.io/underwater-gps/integration/bluerov-integration/

The D1 locator contains a depth sensor which simplifies the integration. It also has its own cable so there is no need to integrate with the vehicles tether. It is possible to integrate the D1 with the vehicles tether if you can spare three twisted pairs of wire.

The maximum length of the D1 cable is 100 meters which prevents the maximum range. 

There is no difference in accuracy between the U1, A1, and D1 locators.