I guess you might be thinking of the Laguna San Rafael - this is the location of a glacier that reaches down to the sea and there are icebergs in the bay (laguna). That would be - I think - the nearest glacier/ icebergs to Puerto Montt, but is still a long way south. I think Navimag ferries do run a service from Puerto Montt that visits the Laguna and this area but this trip takes several days.
The only day trip to Laguna San Rafael I am aware of starts from Puerto Chacabuco and is operated by Catamaranes del Sur - this is still a long day trip, leaving at about 7.30am and returning about 10pm. I took this trip in February and it was spectacular!
To get to Puerto Chacabuco from Puerto Montt would involve a ferry and or a combination of car/ ferry or you could fly to Balmaceda/ Coyhaique and rent a car and drive to Puerto Chacabuco. However, none of these options would enable you to do Puerto Montt to the Laguna and back in one day.
I hope this helps?