The Deben has some charms but the tidal flow plus mud doesn't make it an ideal winter first outing location.
Forgive me but I forget if I've heavily promoted the suitability of West Mersea on Mersea Island for your purpose. I'm guessing 10mls nearer for you than Felixstowe ferry and so much better with minimal tidal flow at the launch point, parking, free slip good for all states of the tide, great pontoon, toilets etc... even the RNLI station on site! It's the perfect location for running in from scratch.
For the very first trolling stages you can turn North on leaving the slip/pontoon and there are two different legs of the creek heading inland that are totally sheltered.
Launching question West Mersea