I am trying to remember the title of a children’s book i read in the early 1990s. It was a short paperback chapter book. The story was about 2 brothers (they may or may not have been truly biological brothers) and they were opposite and did not get along. One was very athletic and the other was nerdy and more brainy. They go on a summer vacation together (grudgingly) to the East coast. I want to say the Maritimes? Somewhere with very high tides. Anyway they go exploring some caves together, but time passes faster than they expected, the tide rises and they are trapped in this cave together. But they find some magical way into a fantasy world. And there are dragons and magic. And the nerdy brother is the one who is more clever and able in this world than the jock brother. And there is a part where i think they have a riddle contest with a Dragon and the nerdy brother outsmarts the dragon. The brothers become friends in the end.