When decking out your seamoth, it's best to just build all the upgrades, keep the 4x4 compartment as a main-stay upgrade always added, and toss the rest of the upgrades in the seamoth compartment. This lets you modify it's load-out on-the-fly while out adventuring. Just cruising up top between bases? Slap on the solar charger. Going deep depth? Slap on a one or two depth mods + one or two of these 15% power efficiency mods. I always keep a spare power supply, battery, and med kit in my seamoth's stowage in addition to the mods. Ever since I started using the solar panel on surface treks I have had to chance the power supply at all. Real resource saver.