Usually, the last thing you wanna see while stranded in the water is a shark ... but this time, it was a GOOD thing -- 'cause "Shark Tank" star Robert Herjavec saved the day by rescuing an ex-hockey player and his family during a boat trip on the lake!!
Here's the deal ... 41-year-old Tom Lawson -- who played 10 years in several different leagues -- was out with his wife and kids on Lake Joseph in Ontario, Canada ... when their boat ran out of gas.
To make matters worse, the sun was going down and Lawson's boat was not meant to be out at night ... so the whole family started flailing their arms and calling for help.
As it turns out, the businessman was out on his jet ski at the same time ... and noticed the family trying to get his attention.