Lahko surfam danes?

1. Vremenska napoved can't "reči", since it doesn't have mouth. So it can't say something, but it can tell you something.

Perfect native feel would be with this sentence:

Vremenska napoved se glasi: Sončno in vroče bo ( Weather forecast reads as follows: It will be sunny later on )

But since you wanted to use sentence as one unity I used word "pravi" in original text correction. Which is closer to "tell" than "say".

2. You could say "Google reče" but  "Google pravi da ja" sounds better.

If sentence is telling us about  "napovedovanje" (predicting+announce) you don't need to add "bodo" (will) to it.  In your sentence it's actually misleading because "bo napovedal dobre valove"  means ( will predict+announce good waves) while in fact that is already predicted and announced right now, but for later on. At least that's how I understand what you were trying to say.

But you could also mean something else. Google pravi da ja: napovedali bodo dobre valove  (Google says yes: they will predict+announce good waves) . Of course in this case you are saying that someone else will predict+announce good waves but they didn't predict+announce yet.

Or you can say: Google pravi da ja: napovedujejo se dobri valovi.  (Google says yes: good waves are predicted+announced)

Just for the side note. Napovedovanje = predictment + announcing  in my way of explaining this word.  Because in my mind it combines both of those two words. If you are really interested I can try and explain why. But dictionary will probably say: announcing/announcment depending on if it's verb or noun.

3. Ne kupim jadralne deske ali neoprenske obleke

It doesn't sound perfect. "Kupiti" is completed verb. It's much better to use incomplete verb here: "Kupovati" since you are talking about your behavior, not your one time situaton, you want to be talking about  something that doesn't complete and therefor doesn't change. If verb doesn't finish it means it still has function in every other comparable instances/situations.  It is above just one time situation.

And since "kupovati" indicates generalization. It is better to use plural. In my opinion it  would  be best to say: "Ne kupujem jadralnih desk ali neoprenskih oblek"

4. Izvedela boste.

that can't be ..you are combining plural and dual.

Izvedeli boste (plural)

Izvedela bosta (dual)

Izvedela boš (singular female)

izvedel boš (singular male)

Maybe in "respected" speach someone would say: "Izvedela boste"  to women, but it's not exactly right way to do it.

Lahko surfam danes?