I’ve always slowly submerged myself into cold pool or lake water until recently. Now I just dive in and get it over with.
You?
I dive straight in (assuming I know the depth of the water). It’s much less stressful than inching one’s way in.
I always tell myself that I’m going to dive right in but…I’m a huge wuss. I just inch in slowly.
Dive in, always.
A bit of both… I stand in the water up to my ankles until my feet feel warm again (or go numb), and then I dive in.
I usually ease in because, hey, I’m in no hurry. Though to ease the pain I’ll splash water on more sensitive areas, like shoulders and stomach, to get them wet early on.
Jump right in! If I try to inch my way in slowly, I’ll just get fed up with trying to adjust and jump in anyway. 
No guts no glory.
Pool: Jump in.
Lake or Beach: Walk in until water is over knees but not up to… there… yet. Throw some water on myself. Plop down the rest of the way all at once.
Dive right in. Seems scary, but it isn’t. I’ve decided I can either say “agh it’s cooooold” 10 times and whine as I ease in toes feet knees thighs privates gut chest and shoulders (and maybe head) and shiver or dunk myself in headfirst and say “AGH it’s COLD!”, shiver like mad and get it over with.
p.s. it’s always cold - why do they always ask if it’s cold
Plunge, although last time I tried this (Lulworth cove, more or less this time last year), it was bone-numbingly cold - I thought it would be a case of getting used to the water once I was in there, but it was a shock taking the plunge, then painfully cold all the time. I think I lasted about a minute in there and after coming out, I was cold for the rest of the day - despite it being hot and sunny.
Guts are one of the things I don’t want in the water, thank you.
I dive right in sometimes. Other times I ease in for the torture factor.
Dive. Best to get it out of the way.
Am I the only one who can’t breath if I jump straight in to very cold water? I find that my chest freezes (figuratively) and I have a similar experience to being winded.
Dive in.
Ease in until the water is just below my crotch, then plunk down the rest of the way.
When on vacation in Maine: Try to ease in, but then have to dive when my brother comes chasing up to splash me.
Susan
That sort of thing is quite common, I believe; apparently, for most people falling unprotected into very cold water, it isn’t the low temperature that kills them directly, but rather the fact that it induces breathing problems and they end up inhaling water and drowning.
Dive in. Alternatively, return later in season when temperature is more pleasant. 
Bomb butt-first into the water - has the added benefit of soaking anyone watching with cold water too