Hi Görkem! Rafael from Blue here.
The main force that will affect the ROV will probably be the drag of the tether cable. Because the cable suffers from a drag force, it will be in an “arch” position from the boat to the ROV, and this drag will also affect the ROV.
To calculate for the drag of a cable, we would usually use the drag coefficient of the cylinder, hence 1.2, but I found a paper (“Measurement and Analysis of Hydrodynamics of ROV’s Tether Cable” - Kojima et al) that better estimates the drag coefficient of tether cables in the water as being around 2.0
With this value in mind, you can calculate for the drag using the drag equation. Keep in mind that the area in the formula is the area of the cable facing the water current, which can be approximated by the length of the cable times its diameter, and that you need an estimative of the velocity of the water current.
As an example, if you were using a cable with 7.6mm diameter, 7000m length, and a water velocity of 0.5m/s, we would have around 13KN of drag. Keep in mind that this is for a very low velocity of the water. With 2m/s it goes to 200KN.
Those are estimatives, but it can be seem that a 7Km cable generates a lot of drag.