That is really something to just experiment with. Depending on the lighting, you may want to try something around 1/8 sec to start with and then start stretching it out from there.
At around 1/8 second, you should get the sort of streaking looking water but still fairly clear. Start going over a 1 second exposure and you start to really get the "cotton" look to the water and the major soft effect. I'm doing this out of memory, haven't shot these types of photos in a while now...
Grab a roll of film and a notebook and then shoot a roll with the same subject and try a ton of different exposures (maybe 1/8th all the way up to several seconds) and record the exposure on each one. Then, after you get them developed, you can decide what exposures give you the effect you are hoping for (and don't forget to post some samples on here!).
I was thinking 1/8 too, until I saw the film speed. This may get tricky because 1/8 is letting a lot of light in on a bright sunny day for an already fairly speedy film. You may over expose. I would normally try 1/8 on about 200 ISO.
Good point on the film speed, I thought about that for a second too!
It is going to depend partly on what the lens aperture is capable of. You may be shooting 1/8th at f32
You should really try a lower speed film (I'd suggest 100) and make sure you are using a tripod. The 100 speed film will give you great options for longer exposures with the possibility of different range of f-stops. In some cases, if it is really bright, you may need to get a filter (I think they are ND filters) that will effectively limit the amount of light coming through the lens and give you the ability to take longer exposures without overexposing.
Am I showing what am amateur photographer I am yet?!
This is true, but I caution against comprimising your desired f-stop. I got caught in this trap and it really hindered me for a long time. You should pick your f-stop based on what depth of field, etc you want....not necessarily because you need to let in more light.