Ive been using Lowrance products for many years and find them extremely handy for both diving and fishing.
With diving and using side scan I can find rocks, caves and even identify some of the bigger fish species like grey nurse sharks. Combining my older hds units with point one I can drop back on the anchor and pin point exactly which side of the boat and how far I have to swim to reach that cave or shark etc.
The newer units have the electronic compass built in these days I believe. If the money allows you can always add on other bits as you go such as side scan etc.
As for navionics charts you could find spots online for free then punch in the gps marks into your own gps sounder to save a few quid. FREE navionics for anywhere in the world Navionics