Ok, I'm going to take all the heat on this voluntarily because I feel increasingly strongly about it.
Let me make some points:
Special Olympics is a great cause. I don't think anyone doubts that. And I'm not going to argue that. I'm not arguing the OP's honourable motivations. I'm not saying no-one should donate. Charity is a necessary part of life. To give is usually better than to receive. Charities protect the most vulnerable in all walks of life when society abandons or ignores those most in need of help.
What is a Polar Plunge? It's not defined. No maximum temperature or immersion duration. As a very cold water swimmer myself, these are not insignificant questions. So it's akin to just asking someone for money, because there are no goals.
But I am not saying that OP is doing that, just contextualising the feat. It really is no big deal and anyone who is healthy and without a history of cardiac problems can do it (just get into even very cold water, without swimming more than a couple of minutes) fairly easily, and anyone who isn't healthy shouldn't do it.
Cutting a hole in the ice sounds great, but that just means the water underneath it is the same temperature as what tens of thousands globally do regularly (daily or weekly). It's probably be between 2 & 5 C. degrees.
But the mechanics of this swim are not my point. My point isn't even about this effort or OP's post, nor whether you should or shouldn't give to this or any other cause. Hell, if you have the money, donate. Ok, so we've established in some of your minds than I a gobshite who hates charity.
I am primarily an open water swimmer, even if 80% of my training is in the pool. One problem we (OWS) face is the growing imposition of mandatory charitable donations for races and swims. As someone who previously fund-raised substantially through my swimming, I now find the imposition both on me, and on others, unwarranted and increasingly unacceptable.
Expected or imposed charitable donations assumes that all swimmers have the same means, and the same motivations. It substitutes a love of what you do for favour of another motivation better valued by society at large. It actually imposes that motivation.
Unlike most other other sports, any time I say I am swimming from X to Y, I get asked "for what charity?". And I know from experience what happens when I say "for none" or "for myself". It doesn't matter that I train year round, that I put in hours in water that will kill some people or scare most people. I've just become the most selfish person in their eyes. It may be because I've had back put against the wall, because I don't have endless means, that I feel so strongly about this.
I swim because I love to swim, and I love to swim open water. I almost can't express how much I love it, nor how essential it is in my life. My 6 degree swim at the weekend is my *mother's little helper". I am not alone.
Why should open water swimming or the thing essential to my mental well being carry a necessary imposition of charitable donation beyond other sports?
Let me give a example. From 2013, swimming around Manhattan, aka MIMS, (~40k) now carries a mandatory charitable donation of $250. In case you are wondering the other costs amount to about $2500 for the swimmer.
Pool swimmers do not suffer this expectation or imposition. An open water marathon swimmer who swims for the love of distance open water is seen as selfish if they don't fundraise.
But let me also tell you, even if you want to fundraise, after you've swim say 40k, asking the neighbours, family or friends for €5 or $10 for swimming 1.5 is a big stretch. Again, I know this. They actually think it's insulting to be asked because after all, they say, "you swam for 18+hours, 30 minutes isn't going to be hard". And they are right. But if I want to do some races or swims, then it's necessary to donate. So our costs go up. So I don't do many of those swims.
Every sport has extremes. But doing those extremes or even doing the simple stuff shouldn't be seen as only for fundraising. Charity shouldn't be a requirement for doing something you love. Charity shouldn't be an externally imposed restriction.
Open water swimming should not become exclusively associated with fundraising.
For those that want to do so, and are successful, (as I was previously) then I salute and commend them. For the rest of us, allow us to make our decisions about for what, how much and when we give.