I bought a soldering iron last week (for antenna PL-259 connectors) but haven't done it yet
Sorry if I misled you with regards to soldering marine wire connections. The only thing that I recommend soldering is the PL-259 coax cable connector. Will need at least 50-75 watt soldering iron (not gun) with a heavy tip approximately 5mm wide. Watch a few you tube videos on how to solder the PL-259 connector; very technique sensitive to achieve a good joint and make sure you test the connector after soldering to ensure that there are no shorts in the assembled connector. (You tube shows you how to do it).
With regards to all other wire connections, I use marine grade mechanical / crimp connectors (Anchor makes quality marine connectors) and marine grade tinned wire. Strip the wires to be joined, thread shrink film over the wire & move it down the wire (away from the crimp), dip the stripped wire in lanocote or other anti corrosive electrical grease, crimp the wire with the appropriate sized connector using a ratcheting crimper, give the wires a tug to ensure a tight connection, and finally pull the shrink film over the crimped joint and apply heat.
If you are going to do your own maintenance and installation of electrical / electronic components, be sure to purchase a good quality ratcheting crimper and a stripping pliers; those tools make the job easier and improve the quality of the crimps. Again, you can watch a few you tube videos for assistance.
Can't offer much advice regarding the depth finder. I usually use the manufacturer's wire and purchase a longer wire from them to make the wire run without splicing, especially if traversing the bilge where a splice could get wet. I agree with Mike's recommendation to have the depth display in the cockpit within sight while at the helm. The display will be useless at the chart table. If you're approaching "skinny" water; need to monitor depth on a continuous basis.