I think the wheel used with your depth finder is the best bet. It doesn't have to be accurate as much as consistent. As long as you can make a reference with it when catching fish, you can keep going back to that speed.

Some guys are going to talk about keeping their line at a 45° angle, but I don't think that's totally accurate. The speed to keep the line angle consistent is going to change with the lure you're using. A bigger, more resistant lure is going to need more speed to produce that angle. As will a heavier ball with a downrigger.

Some use a GPS, but that only gives you speed relative to ground movement. That's not reliable when talking about moving with or against the current.

I think the important thing is consistency with your speed relative to the water's current. That's why I think the wheel for your depth finder is the best way to do that. Whether you're going with or against the current, that wheel will measure the speed you are moving through the water.