I've never heard of "sealing", but understand the concern if you're referring to the drippy sap that sometimes comes out of the cut stems. I wouldn't worry about water sources with callas because they are so fleshy and hold so much water, they could probably stay out of water for two days. What we do is gently squeeze the cut ends, trying to milk some of that fleshy goo out, so it doesn't seep out onto the dresses, or hands. Just be careful not to squeeze too much, or you end up distorting the stem, which doesn't look good for hand tieds. Then we blot thoroughly before packing them up for delivery.
We only occasionally use a water source for hand tieds ... once when the bride was travelling 90 minutes for her ceremony, so these flowers would have been out of water for almost four hours before "showtime". Other than that, we pack carefully and instruct the bridal party to handle them as little as possible before photos & the ceremony.
tracy
