I wonder if you have managed to cycle along the creek, on the north bank yet?
Is this some sort of initiation test? Send a newbie down a dead end? If it is, well played sir, if it isn't, I think it should be from now on.
I'm not 100% on where you exactly mean, but I think you mean the bit between cosham at the portsbridge roundabout, and the eastern road/A27 roundabout at farlington.
I regularly jog through the forest just to the south of the creek, and know that of the two choices there (either side of the three ponds) the north side is definitely very passable on anything other than a road bike/skinny wheeled hybrid, the south side, can be a lot narrowerer/lumpier/muddier, probably not ideal for a tandem.
Where the north side of the pond route reaches the railway tracks, there's a small descent down, with a bit of a tight turn on gravelly wet bits, taken slow though shouldn't be a problem. Tandem may be tricky but certainly passable, may involve a bit of walking/pushing to be sure, but advice would be go there and see.
Now for the path to the north of the creek... (The water separating portsea island from the mainland).
I didn't know this path well so I went and had a look see this morning. Went for the Cosham end first, narrow path very gravelly, didn't fancy it on my skinny wheeled hybrid. Eventually got the Eastern road end and had a look there, ageing concrete path. Barely a foot wide in places due to overgrowth. Get to the railway line and bam. path just stops, well say "stops" what i mean is descends menacingly into the water. In a word. Impassable.
This Time Next Year. Great user name! 
