The order in which I get things:

    - L1 boots
    - L2 boots          (boots help you swim much faster, which is THE MOST important thing to increasing your harvest-rate)
    - L1 laser
    - L2 laser          (laser slightly improves how quickly you can harvest an individual item)
    - L3 boots          (FASTER)
    - L4 boots          (FASTER)
    - L3 laser          (FASTER)
    - L1 air tank       (to be able to stay down for a bit longer)
                            (all this time, you're still staying somewhat shallow...  there isn't
                            too much to be gained by going deep, until near the ned of the game)
    - L2 air tank       (easy to skip)
    - L4 laser          (FASTER)



    - L1 suit           (at this point in the game, you're going deeper than you had before)
    - L2 suit           (deeper)
    - L1 air tank       (deeper/longer)
    - L1 light          (deeper)
    - L2 air tank       (deeper/longer)
    - L3 boots          (SPEED!)
    - L3 air tank       (deeper/longer)
    - L2 light          (deeper)
    - L3 laser          (speed)
    - L3 light
    - L4 light          (the jump from L3 -> L4 is basically free, since glow bulbs are so cheap -- 4 dirt each)



First trip out, you need to get:

    - 2 energy, 2 coal, 5 weed => L2 boots
    - 5 energy, 2 coal, 5 weed => L2 boots + L1 laser