Same for were I come from. The last time I remember using "swum" in a sentence, was in my English classes in school.:-}
Indeed, it is.
It's consistent with how the vowels in some English verbs morph from "i" to "a" to "u". For example, compare "sing", "sang", and "sung" ("I sing", "I sang", and "I have sung") with "swim", "swam", and "swum" ("I swim", "I swam", and "I have swum"). "Drink", "drank", "drunk" is also a common example of the vowel morphing. (This feature of morphing vowels goes back to Indo-European, and can be found in Sanskrit as well, where it's explained as "vowel gradation".)
Not really consistent though, it's just an old quirk a handful of strong Germanic verbs share. I don't know about you, but I've never sut (sit), druve (drive), guve (give), thunk (think, except sarcastically), wrute (write), ruse (rise), etc before. Even when that pattern is sorta followed, the past particulate and preterite have frequently morphed together - like regular English verbs.
I strongly suspect in a generation or two saying "have swum" or "have drunk" will make you sound like a grandparent - if it doesn't already.
I hovered over and it suggested "swam" above "swum", but marked it incorrect, is there English grammar I'm missing between the two words?
In English, you have the preterite (aka simple past) and past particulate. I played vs I have played. For the majority of verbs this is the same word: walked/has walked, barked/has barked, cooked/has cooked, read/has read.
But in true English fashion, some long dead rules linger on as verb irregularity: wrote/has written, sang/has sung, ran/has run. Swim - and drink - are similar: swam/has swum, drank/has drunk.
Note that this is a formal definition, and might not feel right. It's not really an important distinction when speaking, and there's a lot of evidence just in the Duolingo Dutch course that it's definitely not universal when writing.
Regardless, the Dutch moderators have laid down their opinionz on English grammar, and that includes only translating as "My dog has swum in the water."
Amazing, I've learned so much about English while trying to learn Dutch! None of this was in my school's syllabus, I didn't even know there were different types of past tense, I think it's my lack of education in English that's making the grammar hard to learn for Dutch but you're helping me get there one step at a time.