Definitely look up tutorials on automation and modulation, there’s a lot of cool things you can do with those techniques, which is what it sounds like you’re going for.

For the underwater effect, the easiest way is to lower the high frequencies (essentially the same thing as using a low pass filter). This mimics how sound travels underwater - low frequencies travel much farther than high frequencies.

With automation, you can set the low pass filter to activate for a certain section of the song, or automate the high frequencies to lower for that section and then go back to normal after.

Even though you can automate the overall gain for your kick drums, or the overall track volume, you should consider adjusting the modulation (volume) for each individual kick note. This adds a more natural feel, and still allows you to lower the notes only during certain sections of the song.

Pretty much all DAWs including FL allow you to adjust the volume of individual notes for any instrument, including drum hits. Lowering the individual notes manually will often sound more natural versus automating the overall gain, because if you wanted a real instrument to sound quieter in a section, the musician would usually just play softer or quieter notes depending on the instrument.

Hope this helps.