I have to agree with most of what being said on here, especially:
Know your boat. I would recommed taking the boat out by yourself so there is no peer pressure from anybody to go faster or anything like that. Then drive where you feel comfortable at, probably 40 or 50 and just get used to it, then you can progress speeds as your ability increases.
How fast is fast? That answer is relative to water conditions and everything else. Somedays you can get bored driving your boat at 70 because of how flat the water is. On the flip side, 50 can be fast when conditions kick up (4-6' and chop etc). Just take your time to progress.