Through hull fish Finder for depth with verifier accuracy gauge called keel for those locations less than 8 feet deep. For speed take a moment during slack tide/current using your GPS against the amount of wake that you create from 3 to 12 mph to improve your ability to estimate speeds by eye. It is unlikely you will be harassed by a multitude of Slalom skiers for rides and you'll not be replacing repairing through hulls.