1. I've heard you can get yoshi in Delfino Plaza?

1.You need to have beaten one of the Pinna Park stories, I think it's story three.

Shadow Mario will be running around town with a Yoshi egg in his hands.

2. I think you need to swim through it, I don't know what level you are talking about sorry.

3. Use your rocket nozzle I think.

1.) Once you beat Episode 4: The Wilted Sunflowers of Pinna Park...Shadow Mario will appear in Delfino Plaza holding a Yoshi Egg.

Just chase and spray him 'til he falls over (which you will get used to doing as you'll need to do this in every level for Episode 7) to earn Yoshi! Once you get Yoshi...

He will always be positioned on the tower to your left (from the start)...always asking for a different random Fruit...most of which you can find at those stands by the beach on the right side of the Plaza!

2.) I'm not sure what you mean by that hollow standing log in the water...is that in a certain level or is it in Delfino Plaza and if so...where exactly is this log?

3.) If that's the Shine Sprite that appears when you get 100 Coins...then I suggest just waiting 'til you get the Rocket Nozzle (the red box).

You'll get the Rocket Nozzle in Delfino Plaza the same way you'll get the Yoshi Egg...by beating Shadow Mario. He'll appear carrying the Rocket Nozzle once you get the Yoshi Egg and beat another Episode.

Just chase and spray him 'til he falls...then the red box will appear ahead of you (from the start) to the right of the fountain with the Pineapples in it behind the statue/entrance to Bianco Hills!

Good Luck

1. Collect Shine Sprites (10) around Delfino Plaza and Bianco Hills. Go to red dome (cannon) in the northwest corner of the city. Get inside and complete Pinna Park episodes up to Episode 4: The Wilted Sunflowers.
When you return to Delfino Plaza, chase after Shadow Mario at Bianco Square (near fountain), spraying him as he runs across the top of buildings. When he drops the egg, hatch it by bring it the fruit displayed in it's thought bubble. Later on, the egg will appear on the building to the west of Bianco Square.
L to slam into ground, R to spray juice (to remove yellow zig-zags), B to use tongue to eat fruit to refill juice meter, A to jump (hold down to extend jump distance). Yoshis can't swim.

2. Are you talking about those stone platforms in the water, behind the lighthouse? You need the Turbo Nozzle to break through the saloon doors in the water and get the Blue Coin behind it.
Collect Shine Sprites (25). At Delfino Plaza, chase after Shadow Mario near the Boathouse, spraying him as he runs across the rooftops. He runs back to his starting point and drops the FLUDD gadget. Grab the Turbo Nozzle and use R to jet across the ground/sea really fast.
Later on, you can open the grey crate near the fountain at Bianco Square to use the Turbo Nozzle again.

3. If you're talking about the Shine Sprite that appears in Delfino Plaza after you collect 100 coins, you need to wait until the boats start running again.
Collect Shine Sprites (6) around Delfino Plaza to make boats appear. If in sync, jump atop boat at docks, then jump atop boat near western island. If not, just swim to boat going between western island and wooden platform out at sea and climb up side.