Toilets, Sinks, Showers, Tubs and Disposals - Undermount tub waterproofing

We have bought an Americh bathtub model Madison II 6030 to replace an old stadard alcove type tub that we had based on its specific size and heigh. The new tub can be dropped in or undermounted so it does not have tile flanges typical for alcove installation. We need to keep a shower on the wall but a plumber told us that no matter how we waterproof/install the tub it will leak between the tub and the walls bacause of lack of flanges. However, I saw many photos of the dropped-in or unermounted tubs with showers on the wall and our friends have jacuzzi with no flanges and they say the water from their shower doesn't get behind the tub. Can anyone share their experience or tell us what can be done to prevent water leaks. Please let me know if I did not described the problem properly. Thank you.