Watanabe began baseball at age 6, and began throwing underhanded during middle school at the suggestion of his father. Watanabe attracted little attention through high school and college, and joined the Kazusa Magic amateur baseball team in 1999 after graduating from college. He was finally noticed by professional scouts when he was chosen as a member of the 2000 Sydney Olympics Japanese national team, where he marked a win in a game against Italy.
He pitched in the Japanese national amateur baseball tournament in Autumn, 2000, and was drafted in the 4th round by the Chiba Lotte Marines that year.
Watanabe made his debut in April, 2001, starting a game against the Orix BlueWave. He won his first professional game with a complete game shutout, and ended the season with 2 wins. However, he only pitched in 6 games in 2002, and had a record of 0-3. In 2003, he gave up 8 runs in his first start, but became a part of the starting rotation at the middle of the year, going 9-4 with a 3.66 ERA. He won 12 games the following year, and truly made his mark in 2005, when he won 15 games with a 2.17 ERA. The Marines won their first championship in 31 years in 2005, and Watanabe pitched in the second game of the Japanese championship series against the Hanshin Tigers, giving up only 4 hits in a shutout victory.
He was chosen as a member of the World Baseball Classic team in 2006, but pitched poorly during the season, ending up with a 5-11 record, and a 4.35 ERA. He also led the league in hit batsmen (14).
Watanabe is known for having the world's lowest release point, letting go of the ball only about 2 inches above the ground, and his hand sometimes brushes against the ground as he throws. His body is much closer to the ground than other submarine pitchers, and his right knee has bled during games because it skids against the mound. He puts a pad on the inside of his uniform around the knee to prevent himself from bleeding. Watanabe's form is truly one of a kind, as there is no other pitcher in the world that is able to throw from the same arm angle. Batters have trouble timing their swings against Watanabe, because his pitches seem to come in at completely different speeds compared to those of conventional pitchers.
Watanabe relies on his distinct pitching form and timing to get batters out. In addition to changing speeds with his pitches, he sometimes changes the time he takes to windup and release the ball. He does not have precise control, but is consistent in the lower part of the strike zone. Watanabe is not known to strike out batters, (only 101 strikeouts in 187 innings in 2005) and relies on forcing batters to hit themselves into outs. His underhanded form requires less energy than a conventional pitching form, and he is always able to pitch into later innings.
Watanabe throws four main pitches; a rising fastball, slow curve, sinker and slider. Being a submarine pitcher, Watanabe's fastball is very slow, falling in the high 70 mph or low 80 mph range, but has lots of movement. The sinker is probably his best pitch, as he fools batters by throwing it at the same speed as his fastball, or sometimes even faster. His slider is only slightly slower than his fastball, but breaks instead of rising upwards. Watanabe's curve is painfully slow, clocking in the mid 50 mph range, somewhat similar to the curveball of New York Mets pitcher Orlando Hernández (which actually is more like an eephus pitch). He often experiments with the curve, changing grips to make it resemble a changeup more than a curve. This curve has been recognized as a two-seam Gyroball.
- He holds the Japanese record for skipping stones, recorded on a show on Nippon Television. Chiba Lotte Marines manager Bobby Valentine made an appearance on the show as well.
- David Ortiz of the Boston Red Sox hit a 525 foot home run off Watanabe on November 6, 2004, when the United States Major League Baseball team faced the Nippon Professional Baseball team in the second game of the traditional Japan All-Star Series. This mammoth blast by "Big Papi" has been recorded as the longest home run ever hit at the Tokyo Dome.