I love my myerchin L300 (~$70), I find it easy to open with one hand, locking marlin spike and a red led chart light that doesn't spoil your night vision. I do not consider the slot in the blade a proper shackle key and would never use it as such.
I have a separate heavy duty stainless shackle key / bottle opener on the same halyard but find I don't use those that much (not many shackles on my boat and I generally don't have bottles onboard cause I'm allergic to broken glass).
I use the light
way more often than I thought I would, more often than the knife or spike actually.