It's not a fishing game. You need to gain weight to move down. It's in the initial tutorial text.You have to suck in the goldfish by getting close to them, so as to gain weight and sink deeper or drill-kill piranhas before they kill you.
the harpoon always falls short
That's actually by design. If it were longer, it would've been less challenging for players to suck in the goldfish or drill-kill the piranhas.
