As stated you can walk it. I've done it once and would not do it again without good reasons.
The Disney Resort Line is very easy and fast an only cost ¥250 for a one way trip. If you plan to ride it multiple times in one day (more than two) it's cheaper to buy a one day pass. There are also multi-day passes for sale.
If it's worth it for you to make a 15-20 minute to save around $3 then walk.
Take The Disney Resort Line... it offers great views and a behind the scenes look of DisneySEA. If you are lucky you will be able to get a quick glance at Mt Fuji. I think it is seen from the right side of the train.
If you feel like walking you will get what you want in DisneySEA.
And sometimes you get lucky and a character is waving at the guest in the monorail from backstage!!!
Bringing the Love since January 10, 2011
We were sent over there with the message that only we would defend Disney quality.. We learnt quickly that the Japanese culture speaks to a level of quality that is hard to comprehend. They were all unbelievably skilled and willing to do the right thing.
Craig Russell -Walt Disney Imagineering
We took the Resort Line in May, and my daughter saw Mickey, Minnie, Duffy, and Shelley Mae backstage walking near the Tower of Terror. They all turned around and waved at the monorail. She's 10 and a half, and knows Disney parks like the back of her hand, but she was still dumbstruck.
As far as walking between the parks, it's not a "nice" walk. The sidewalk is fairly narrow and it's on a busy street. Defniitely out of the Disney "zone." A monorail Pass (resort line) is pretty cheap for however many days you're going to stay. Even if it's just for one trip between the parks, consider it $3 for an attraction, because that's really what it is. Going from Tokyo Disneyland to DisneySEA you can really appreciate how carefully the monorail and track are concealed from people in the park.
I've done the walk. Agree that it's not a great "walk"; it's not designed to be; just a sidewalk. I just wanted to see what it was like and see if there was anything backstage that I might see. Mistake. Maybe I wasn't looking hard enough, but did not see anything of note. Also at the end of the day, it's pretty busy w/ buses.