You are in for a treat, Negril is great! EVERYONE has herb, it's all sticky icky, fresh and mostly organic, even the brown weed will have crystals and be tasty. Quality and price will vary,
you have to know how to negotiate with these people. Don't be scared to haggle with them, it's what they do. $60 could get you a few joints or a Z. Everyone says they have the best weed, so you gotta know what you're looking for. If you're staying in the bloody bay area, avoid the guys who have shops between the 2 RIU resorts. All the weed they ever showed me was brown, over priced and they tend to be kind of aggressive. The guys who sell walking up and down the beach are really chill, and have the best weed.
Your dad is kind of right though. There is no such thing a sour d or kush down there. It's all pretty much land race plants (still killer). The only name brand weed I heard of there was g13. But I don't think that's what it is, they just caught on that Americans like fancy names for their bud.
Mushrooms are 100% legal in Jamaica as I understand. I have seen people on the beach selling them but never bothered to check em out. There is a place called Ted's where you can go (take a red plate taxi, like $2) and Ted will make you some shroom tea either to drink at Ted's or take back to your hotel.
The pic above was taken on the beach and it warms my heart. On what other beach can you be on enjoying a frozen drink while negotiating the price of arm length buds?
Have a great trip. You can PM if you have any other questions, I'll try to get back to you.