I haven't noticed any performance issues underwater. It might depend on which mods you're running.

Bloom requires HDR because it needs to know which parts of the screen are really bright. Without HDR, things are limited in brightness, so bloom would not work effectively. Full HDR rendering is quite intensive if you're using integrated graphics or your CPU is a few generations old (thus having worse single-threaded performance), and adding mods makes performance even worse. If your PC can run the high preset smoothy but struggles with the ultra preset, there's nothing to worry about. Your PC might run large mod packs perfectly fine, but the point at which you start performance issues depends on the FPS limit you set as well as how much performance headroom you have. However, if your FPS "disappears from existence," that is more concerning, but that doesn't tell my how many FPS you're getting or what your computer specs are, so more clarification would be helpful. I'd also suggest comparing performance without other mods vs. with other mods.

I don't think lighting resolution would have much of a performance impact, because computing the lighting isn't that taxing on the GPU and the pixel shader to apply lighting to tiles would need to run for every pixel regardless. The default preset already computes only one light color for each lighting, but bilinear filtering makes the lighting appear smooth. Bilinear filtering is very cheap for GPUs to do, so the performance gain would likely be negligible. The bicubic upscaling option computes 16 light colors for each tile and then uses bilinear filtering, so even there the resolution is reduced.

The fix for Infernum DoG is not specific to that boss fight. There is no code in this mod to check if that particular boss fight is occurring; instead, that boss fight disables screen shaders, which caused bugs before because the mod wasn't properly checking if screen shaders were enabled. I will see if I can add an option to disable HDR during boss fights.