I've been wrong once or twice, LOL......but I'm pretty sure MasterCraft moved to 100% fuel injected GM engines in late '94 or '95 for all boats they produced and Ford dropped it's relationship with Indmar. Really odd that yours has a carb. Here is more detail: "SportStar 19 (1998-2000)
MasterCraft introduced the SportStar 19 midway through the 1998 production year as a closed bow model. MasterCraft developed this model as an entry level boat that set new standards for both quality and performance.The load capacity for the 2,400-pound SportStar 19 is eight people or 1,095 pounds with a fuel capacity of 31 gallons.
In 1998, the SportStar 19 came with the EFI 260 (5.0 L) horsepower engine as standard with a 13x 13 three blade OJ propeller. The engine option of 330 horsepower (LT-1) was offered in 1998.The 1998 model included a black wrap-around windshield frame, unpadded Driver’s armrest,bare fibreglass engine cover, one-colour interior accent, non-removable swim platform brackets,and did not have a trunk. A boat cover was offered as an option in 1998. In 1998 exteriorgraphics for the SportStar were similar to those of the 1995-1997 ProStar"
OK, history lesson over...I would drain the oil, leave the filter and refill with 4 qts of 15w-40. Then run it and watch the oil pressure. Then check oil level. As long as the oil pressure doesn't continue to climb and the oil is within the "safe" zone on the dipstick, I wouldn't worry about it. It could have easily been overfilled, and with 20w/50 could cause a slightly higher reading.