(Reuters) -Neptune Insurance said on Monday it was targeting a valuation of up to $2.76 billion in its U.S. initial public offering, as the IPO market continues to run hot. Some existing shareholders of the flood insurance provider are targeting to raise up to$368.4 million by offering 18.4 million shares priced between $18 and$20 apiece. The St. Petersburg, Florida-based company joins a growing roster of insurance firms that have tapped public markets since May, with some calling it a "breakout year" ...