Searching for old shipwrecks made of wood in salt water is best done by using a magnetometer and looking for the iron items on the ship such as anchor and chain. Wooden ships fall apart due to the teredo worm that borrows into the wood and eventually the ship falls apart. Searching for metal shipwreck in salt water at depths of 80m requires a towed sonar with 100m or more cable. Proven Side scan sonars in the $10K to 20K range would be Deepvision, Imagenex, and JW Fishers. See my review on the DeepVision DE-340 on this website. Other higher end side scan sonars would include EdgeTech, Klein, and Marine Sonics which are priced in the $30K and up range. Sorry for the delay in responding as I have been in the mountains hiking and out of email contact. Hope this helps.