Title: Leonard Visits the Ocean Floor (Time Machine Series)
Description: Leonard is a little boy who builds amazing things. A time machine in one book, a submarine/underwater globe? in another.  I remember these books coming with a record of a woman reading aloud in a very dramatic voice. "Leonard worked. Leonard worked and worked. Leonard was building his time machine."  Or "Leonard to deep sea diver, Leonard to deep sea diver-here I come to help! Here I come to help!"
That last one was Leonard rescuing the deep sea diver from some octopus or squid. Very exciting pictures, with repeated words making it easy for a beginning reader. It was for first grade, as I recall. Also exciting, as he managed to do all this adventure all on his own. I would love to see these books again!