I first toured Izu peninsula by bicycle in August during Obon week, a Japanese celebration of ancestors. It was indeed hot and humid, and the experience was magical. I have been back to Izu dozens of times since then, in all seasons, and I continue to be amazed by the seemingly limitless opportunities for the water enthusiast around nearly every turn.
The shape of Izu is a snake's head, tipped slightly to the left. Osezaki, [大瀬崎] on the top left tip is about 22 km southwest of the city of Numazu, a right turn off route 17. It is a scuba diving point and offers up a magnificent view of Mount Fuji. Divers can pay the nominal shrine entrance fee and dive to the right, where there are soft corals and a variety of smaller fish, or they can walk over the hump and dive to the left for a deeper dive with larger animals. On both sides the current can be strong so be aware of distance traveled and where to turn back; use your computer or count kick cycles. After your dives, the restaurants back at the beach area, where beginner dive classes take place, are reasonable and the food is delicious. There's also a hot bath further back in the dive shops' area if you're feeling chilled.
Going south about 10.5 km, you will pass another beginner scuba diving point, Ita [井田]. The road then becomes elevated with some hair-pin turns. Be sure to take the right turn-out just before descending to Heda [戸田] for the best view on Izu. It overlooks the town below and if you happen to arrive at sunset then it's going to be spectacular.
After another 15.5 km, you will pass Toi [土肥 ], a ferrying point across the bay to Shimizu City. Here the road becomes route 136. Keep right and continue on just over 11 km and to your right is a little-known gem, Ukusu [宇久須]. You can't see this rocky inlet from the road, but turn off the main road and make your way down there, staying to the left. Be sure to pack snorkeling gear. You can also rent it from a dive shop there or buy some cheap stuff from a refreshment stand. This picturesque and protected little cove has arguably the clearest water on Izu. I have spotted large squid here, who spotted me back with their arcane intelligences.
12 km further south is the town of Matsuzaki [松崎], my favorite for overnight stays. It has amenities such as an inexpensive grocery store, and is quiet other than for a rollicking beachside fireworks festival in August. There is a hostel just east on route 15 about 1.5 km behind the town, off the main road to the left. The inn is rustic, the rooms are traditional, and the Japanese-style meals more than you can eat. There's also an intriguing display of Japanese relics, such as koban, old-time rectangular coins, in the glass case near the check-in desk.