I am the transportation to PI expert for some reason.
If you are at a park and you want to catch a bus to PI - well, you can't. There is no direct transportation to Downtown Disney from the parks. Here is what I do.
If you're at Epcot, walk out the International Gateway to the Beach Club and take a Downtown Disney bus from there.
If you're at MK, walk to the Contemporary and catch the bus there, or take the motor launch to GF and catch their bus. If you have a thing for the monorail, ride it to the GF. If you get off before the GF, when you catch the bus, it will stop at other resorts before heading out to DTD.
If you're at MGM, that one is more difficult. Fastest way is to take the walking path to the Boardwalk and catch the bus there. You can take the boat - if you do that, get off at the Swan/Dolphin, walk over to the Boardwalk and catch their bus to avoid multiple bus stops. Or, you can catch the first resort bus that comes along and transfer at the resort to the DTD bus. You might end up picking a resort that has lots of bus stops, though. I think the best resort for this would be Pop Century - it's centrally located and the bus will go directly there, and the DTD bus from there will go directly to DTD without stopping anywhere else.
If you are at a resort and the parks are closed, take the bus from that resort to Downtown Disney. If you're headed to PI, you're there. If you're going back to your resort, transfer to your own resort bus either at the marketplace or PI.
Buses run to and from PI until 2 AM, so you're not going to get stranded.
You can't see the water pageant from the Boardwalk, you would need to be at a Magic Kingdom area resort or Fort Wilderness.
BTW, if you want to spend more than a couple of nights at PI, look into a PI annual pass. It costs around 50 bucks. I don't know if they still have the deal where you pay a bit more on your one-night pass and get some extra nights or not.