Hi Frederik,
I'll try and explain using very direct translation.
"Wanneer hulle terug seil na die nette" - When they sailed back to the nets (fishing nets)
"vind hulle dat dit vol duisende derduisende sardientjies is" - They found that it is full of thousands of thousands of sardines" (type of little fish)
Kaptein Bruinders gee die teken om die anker te lig - Captain Bruinders gives the sign to raise the anchor.
"Elke visserman moet 'n lisensie he, al is dit net vir pret"
Okay first off - "he" should be spelled "hê" (use keyboard: alt 136) meaning: to have
Using this together with "moet" is irrelevant. It has got something to do with the tense rather. You also cannot use "het" in this case.
Personally I don't like to use the word "hê" it is almost like slang. I would have used the sentence as follow: Elke visserman moet 'n lisensie besit.
It just sounds way better.
"al" in this case means "even though"
Every fisherman needs to have a license even though it is just for fun.
Now this sentence is probably not constructed quite correctly, because, is the writer referring to the license being fun or the fishing being fun? I think what the writer wanted to say was that even though you are only fishing for fun and not for commercial purposes you still need a license.
Hope this helps a bit.
Take care