I did the trip in February - CC to Clear Lake and came up the ditch. I never saw depths under 8 feet except in an anchorage off Matagorda Island. The only "downside" of doing the ditch is only do it in daylight. From Port O'Connor to Galveston the tow traffic increases and you wouldn't want to pass a tow on one of the turns at night. I've done it offshore too and there are unlit rigs. If you do it offshore do it under a full moon (like the Harvest Moon Race) so you can see the unlit rigs.
Go to the TMCA website that has distances in the ditch and offshore. Under Cruising go to Destinations.
As for marinas I can't say that Seabrook is any safer than Watergate. I keep mine at Seabrook and it's not gated either. The one advantage at Seabrook is in the winter months the tide is lower and in most areas of the marina it is deeper than Watergate - i.e. you could get stuck in Watergate and not be able to get out and sail in the bay because of low tides.