there ain't no ducks down there...better off hunting the tillamook bay.
the quick answers:
hunt high tide
pressure eases off after opening weekend
sea ducks are possible down towards astoria, but not common up around the aldrich point (brownsmead) area.
now, please take time to read the following...sorry it's long winded.
opening day down there is a zoo. unless you want to get down there thursday night and sleep in your boat in the spot you want to hunt in until saturday morning...-or- you know the place well enough to have a 6th and 7th choice hunting location if there are people in the normal places you hunt. you are probably better served hunting the opener someplace else.
are you referring to the dairy farm next to the aldrich point launch? the farmers there are very nice people. good luck hunting those fields. we see some geese in there from time to time and also looks like you might get some decent duck shooting in there too.
now..to the REAL question. after the opening weekend the place settles down quite a bit. there are still a few hunters out there, but it's a big enough place to support more than a few groups out there hunting. The local birds get smart real fast..and the hunting can be kind of slow until some northerns arrive, but spend some time down there and you're bound to get into some birds.
i can't stress this part enough. SCOUT SCOUT SCOUT!! you may have some good days in the marsh, but if you want to be consistantly successful down there you need to know what the birds are doing and when they do it. there is probably 100 square miles of waterway to work with and all of it looks "ducky". problem is..the ducks are only using a small percentage of it at any given time. the rest is just a lonely day waiting to happen. what i can tell you is that the birds do move with the tides. they move up into the islands at high tide to feed on the seeds that get floated when the water moves in...and move out to shallow protected bays and mud flats at low tide to rest. the time of day has no real bearing on this movement. ie: first light might not be the best shooting period if high tide is at 10am.
lastly, it sounds like you are new to boating in that area of the river. please take this seriously. the tides can leave you stranded in a hurry out there. you may be in an area with enough water to move you around, but the surrounding areas will be too shallow to navigate until the following incoming tide. dead heads (big logs) are everywhere and usually just below the surface just waiting to eat your boat. winds come up quick and can make the calmest of waters turn into some nasty 3foot rollers. your 18footer should be more than enough boat to handle this, but still..be warned if you travel "heavy" that this is a real problem. get a GPS. the fog is nasty and getting around down there without one is next to impossible. as much as my hunting partner and I are familiar with that area, it took us 2 hours one morning in the fog to find our blind that is normally a 15 minute run from the boat ramp. (our GPS unit crashed and we were in the soupy AM fog that is very common down there.)
hope this helps..drop me a PM if you like and maybe we can hook up for a hunt and i'll give you a little first hand knowledge that i won't post in a public forum.
Friends Don't Let Friends Shoot Mossbergs.