Douro River cruise
 

We would like to go on a one day Douro river cruise with stops at wineries- any suggestions as to which outfit to pick? (Aug/Sept)

Here are two options, departing from across the river in Gaia, cruising upstream as far as Régua or on to Pinhão and the return to Porto by bus.
https://www.cruzeiros-douro.pt/en/1-...ise-with-visit
https://www.cruzeiros-douro.pt/en/1-...rto-with-visit

These cruises make only 1 stop at a winery; otherwise, it would make for far too long a day. The cruises that run to Régua depart at 9 am and don't return until 8:30 pm, Those that run upstream all the way to Pinhão start at 8 or 8:30 am and don't return to Porto until 9:00 pm.

But before booking, you may want to check the TripAdvisor reviews.

You may want, instead, to visit the Douro on a day tour via small group minivan tour or private tour, during which you can visit more than one winery, have lunch, a short trip up to a scenic viewpoint and a short, 1-hour cruise from Pinhão upriver to Romaneira.

Second maribel's suggestion to think about a small group tour to the Douro valley. We booked through a wine club and wound up using efun tours and were delighted - small group, comfortable minivan, three wineries and lunch at one. Friends who did the river cruise option said it was boring after the first bit of time.

We took a small minibus tour in June with a one hour cruise and I think I would be quite bored with a whole day cruise of the Douro

Just a hearty second to what Seamus and mjs have mentioned---in addition, the scenic part of the Douro river sits between (Peso da) Régua and Pinhão and the most dramatic, from Pinhão upstream. So on a day long river cruise just as far as Régua, you wouldn't have the opportunity to see it at its very best from the river.

As Seamus notes, some of the small companies like EFun offer a visit to 3 wineries and typical lunch at a vineyard, which to me sounds like much more fun.
It's something a day long river cruise can't do since breakfast and lunch is taken on board (quality of dining may not be very impressive) and there's only time for 1 winery visit.