Well, I must say you all seem to be quite tough on your kids- they take the brunt of your devil may care, entrenched attitudes.
I would never insist in wearing something my dcs asked me not to. Why would I? They have to live with the decisions we make as a family- they don't just follow along in my blaze trailing, right-on wake like cardboard cut outs going up in flames. They have lives and feelings and opinions as valid as my own. If they think black from top to toe is too much they'll tell me, and rightly, and I'll change into something a little less funereal to accommodate them.
No man is an island.
I think this 8 year old will remember her dad's speedos, and the teasing she got because of them long after he's gone, and long after she's grown up. This is what memories are made of.
If he's too bull headed to change into something that would suit him better and make him look less ridiculous to facilitate her, he's a bird brain. (I'm assuming the OP's DD has asked him)
It might even put her off swimming.. Or him.
That's a lose lose situation, and all because, yes, he shouldn't have to change his swimwear, he's entitled to wear what he likes, yes, she shouldn't be teased, yes, he should be able to wear tight and revealing swim attire to a children's class, even though he's "let himself go", and maybe it's not appropriate.
Sure all those things are true, but she'll remember every cringe-y moment of her swim lessons, the teasing, and how her dad put himself first, and wouldn't change into something a little less conspicuous.
I don't think his head's on straight, and he's missing the big picture.