Brief update: back to work on the game after a couple weeks of hiatus. I've decided to use water in this second mission; said water gives a player an air meter to contend with (at least until/if they pick up the Scuba Power-Up or some-such) and imbues them with a "floaty" jump (think Mega Man).
In implementing the "floaty" underwater jump in Game Maker 8, which involved tweaking both the gravity in the step event (but only when a certain variable was triggered by colliding with my deep_water object) and the vertical speed in the jump event (your jump speed is -4 instead of the usual -5, which, combined with the lower gravity acting upon you, gets you higher up but at a slower speed). In implementing this, and, most importantly, the constantly-resetting alarm events that turn this special jump off, I realized that I could approach this two ways: one way was solid and elegant, ensuring that the player only floaty-jumped when I wanted them to; the other way was an inelegant work-around that allowed them to get to places they weren't supposed to be. Seq.Breaker being the name of the game, I chose the latter, and it was kind of fun to create the coding equivalent of a Rube Goldberg machine-- i.e., going out of my way to make it more complex than it needed to be and intentionally buggy.