How do I choose between a neutrally buoyant and a standard underwater cable?

It depends on where the cable has to sit in the water column. A neutrally buoyant construction is used when the cable must follow the vehicle or the structure without loading it — ROV tethers, towed arrays, dive support. Where the cable is fixed, armoured or laid on the seabed, a standard water-blocked power construction is simpler and carries more copper per millimetre. Density is a design output, so tell us the working depth and the acceptable in-water weight and the construction follows from that.