We are looking to do the Danube from Passau to Budapest by ferry. Our itinerary will change greatly based on whether or not the ferry dock areas have lockers for luggage. We want to stop at Melk, Durnstein and Krems and feel can do each in 1/2 day but would need a storage locker for our carryon. I wrote to the ferry company but since operate April to October don't know when I'll here back. Thanks.

Just in order to understand your question:
do you want to go with a Wurm and Koeck boat up to Melk ar 14.40 and leave Melk at 16.25 with a DDSG boat?

I think my ferries are all DDSG?

I would look closely at your plans and offerings by DDSG.
You can go from Passau to Vienna using the Kaiserin Elisabeth with an overnight in Linz offered by DDSG
This boat while relatively new and comfortable, has no overnight facilities - You can leave baggage etc. on board while touring the several stops .
But I do not believe this boat makes a stop in Melk. This is a 2 day trip , a stop in Durnstein but not Krems.
Other DDSG boats from Melk make stops in Durnstein and Krems.
Durnstein no lockers
.
Melk and Krems have luggage lockers at the train station. Both are not near to the boat dock.
I believe Melk Abbey has small lockers for visitors for a fee.

I knew I could count on you for an answer. Thank you.

Sorry, you spoke about ferries and about hop on - hop off whereas DDSG offers a (by far not daily) cruise from Passau to Durnstein or Vienna for 210 EUR/pax.
It will of course be easy to leave the boat at Duernstein, to sleep there, to visit Melk and Krems on day 3 and to go finally to Vienna with a sheduled DDSG boat (for some 30 EUR more/pax).
For the Vienna - Budapest leg, DDSG offers only hydrofoil trips, however.
http://www.ddsg-blue-danube.at/eng/s...sau_vienna.php
http://www.ddsg-blue-danube.at/eng/c...o_budapest.php

We saw those trips bit wanted to stop at Melk and Krems and it does not.Thanks

"We saw those trips bit wanted to stop at Melk and Krems and it does not"
The Wurm and Koeck boat and the local DDSG boats do, the DDSG excursion boat (which is the most expensive one, BTW: 210 EUR/pax) doesn't.