YA - Kids solving a mystery on an island

[Becky]

My memories of this one are pretty vague, but I'll do my best. The kids live near a river or lake and travel by boat over to a deserted island, where they solve some kind of mystery. I think the boy had a bulldog and there was a horse they called Mister E. I have a ridiculously strong memory of them sitting around a campfire with some adult who scoffed at feeding the dog dry food and compared it to eating nothing but crackers. It's from the 70s or before, but that's all I remember.

[Kim]

hi becky ! so i'm quite sure this is not the book you're looking for, but anyway, your description immediately made me think of Five on Treasure Island. however, i myself read that book when i was a kid so my memories of it are very vague too (tho i'm pretty sure there is no horse involved...). hope you'll find what you're looking for :)

[Rainbowheart]

Pirate's Island?

Surprise Island?

[Tab]

One of the "Steve Forrester" books? He has a bulldog
https://www.goodreads.com/series/2366...

Did you read the book in the 70s or later?
Part of a series?
Anything about the cover?
Time period story takes place?