You might be able to get a fast boat (ie smaller speedboat) but it is doubtful and probably not a great idea - not sure why you would skip travelling in Nigeria (crazy drivers but great country) to brave the waterways of the delta where you probably run a greater risk of danger.
Plenty of fast boats or the overnight ferry from Calabar to points in Cameroon, but I would say that traveling by water through the delta and past Bonny Island, Brass Island, etc. is a really bad idea.
I traveled by motorcycle a month ago from Calabar to Lagos via Benin City avoiding Port Harcourt and had no problems. I spoke with expats and even locals who have been all over the country many times who wouldn't go to Part Harcourt on travel on the delta waterways right now - and these are some seriously hard-nosed Nigerians and Nigerian-experienced expats, both black and white.
The ferry from Tiko, Cameoron to Calabar was boarded and robbed by pirates the week before I took it, and even with a Nigerian navy escort, pirates attempted an assault in the morning (but were scared off by a few warning shots.)
I'd say stick to the roads - I'd take busted Peugot 504's at F1 speeds any day over heavy artillery and hijacking.