Mike Piazza said all but one of his swings yesterday was “lousy.” Obviously, he makes his good swings count.

“Fortunately, I didn’t try to pull the ball,” Piazza said of his grand slam that helped lift the Mets to a 5-1 win over the Marlins at Shea. “Thank God I had that one swing. It was the only good swing I had all day. Every other one was bad.”

Marlins starter Brad Penny kept the Mets quiet until the sixth inning. But with runners on second and third and one out, Penny fell behind Edgardo Alfonzo 3-0 and Florida manager John Boles, who claims that he is not a masochist, opted to walk the second baseman. That brought Piazza to the plate with the bases full. Bad move.

“Right there, you’ve got to pick your poison,” Boles said of his fateful decision. “You’ve got to hope Piazza hits into a ground ball for a double play. But he didn’t.”

Not even close. Instead, Piazza took an outside fastball and drilled a line drive just over the right-field fence, 364 feet away.

“He hit that ball off of one foot,” Boles marveled. “He’s so strong, he can do that.”

It was Piazza’s 10th grand slam of his career and his 10th homer of 2000. By the end of the day, the catcher was hitting a robust .381. Still, both Bobby Valentine and Piazza understood the move, especially since Piazza had struck out and grounded to third in his previous at-bats.

“[The count] was 3-0 with the infield in,” Valentine said. “[Boles] took a chance. He saw what happened earlier and he hoped the same thing would happen.”

Piazza concurred.

“In that situation, I probably would have done the same thing,” said Piazza, who grounded into a double play in his final at-bat. “I just tried to stay composed.”

Despite the fact that he hit the ball as well as he did, Piazza didn’t think he got enough of it.

“Not really,” said Piazza, who then admitted that he pulled a Rickey Henderson- well, kind of. “Since Fonzie was on first, I wasn’t really running. I saw him standing there. I knew it would be at least a sacrifice fly.”

But it was significantly more than that. It was the kind of hit that will likely make Boles think again before he lets Piazza hit with the bases loaded, regardless of the situation. And although Piazza insisted that people had been walked to get to him in the past, he admitted that it wasn’t something that he wanted to see much more of.

“You can’t afford to really think about it,” Piazza said. “But it’s happened a few times. If it starts to increase, though, then I don’t know. Then I’m really in trouble.”