Although I have been detecting for many years, this has been my first season with the Excal and it would be my only choice of the detectors that you have listed if you are going in up to your knees in salt water. I have the F75 and it would probably be my last choice for the wet strip and forget about taking it in the water for obvious reasons. In a field or relic hunting, it would be my first choice. I had an MXT and it is a great detector too, but not for what you are looking to do with it. I do not have a DFX but it would work the wet lane better than the F75 or MXT due to it is a two frequency machine. You don't want to have any of the above except the Excal slip away from you while trying to retrieve a target with your scoop in any amount of water depth. If you are going to change your mind and stay in the dry sand, any of the above will serve you well. I have not had the opportunity to try a PI, so I have no opinion to share. You didn't mention where you will be working in knee deep salt water. If you are working the moving surf in knee deep water a lot of the time, I would go with the Excal 800 because it is easier to handle with the waves pushing and tugging at the smaller coil. If you are mostly going to work the wet strip but seldom go into moving surf, I would go with the Excal 1000 for more coverage. If you are going to be working mostly still, knee deep water, I think the 1000 would be fine. Although you didn't mention these, the CZ series of Fishers are great beach detectors too (mostly out of production), except 3D, but are not units you would want to get wet unless they are the in water version, the CZ20 (out of production) or the soon to be released CZ21. The salt mist in the air can have an affect on parts over time. With the Excal, you give it a good hosing off and your good till next time. I love it, and even more so than many land detectors. If you go with the Excal, look up ......Mike (Virginia Beach) in the search area for a lot of good information regarding settings and tones that he shared with us.