Cavitation.
 

Spent my first weekend with my (new to me) 95 18' Talari w/ the 4.3 v6 4bbl. I know it has an aftermarket prop but I don't have the specs on it, just that it's a 3 blade aluminum. I can say that lightly loaded she'll hit near 50mph at near 5k rpms if that helps. Just a great boat w/ only 150 hours on it. The one problem i've noticed is that with 7 passengers (5 adults, 2 kids) and pulling a tuber it really struggled planing and with some aggressive cornering there were huge cavitation problems.

After doing some research I've ordered a stainless 4 blade 20 pitch prop. Hope this solves both problems. Any other ideas?

Re: Cavitation.
 

The 4 blade - 20" will help but you may have to consider adding a pair of Smart Tabs. Cost is less than $150 & its a 30-45min DIY job. Do a site search for "Smart Tabs" and you will find lots of threads discussing them. They are a very good addition to 18-22' boats and the Smart Tabs SX with 60lb actuators will work best for your boat. You can also visit their website here as there is lots of information available.

As an aside, you must be getting close to the boats capacity with that number/weight of people aboard and with trying to pull a tuber its no wonder you were having some blow outs when cornering. Remember that capacity includes the weight of all the little extras (anchor, gear, coolers filled with drinks etc) that you are carrying onboard & with the additional strain of hauling a tuber (significant resistance in the water) you're asking a lot of an 18' boat.

Re: Cavitation.
 

Also, weight distribution will make a big change in the boats performance. The more weight in the front, the faster it will come up on plane.