If you are in a river that does not freeze more than a few inches you will probably be fine. Your description on the boat you saw in 3-4 foot of ice said that it was pulled out in January. If the hull does not get crushed in that much ice I can assure you that the ice will move in the Spring, especially if there is a current. Windage alone will move ice which will then do major damage to anything in its way. I have seen steel boat lifts turned into pretzels by spring ice.