Norwegian Cruise Lines isn't back on U.S. waters yet and it is already warning of possible choppy seas ahead. The cruise operator on Thursday warned that plans to resume U.S. sailings by the middle of the summer could already be in jeopardy, one day after the U.S. Centers for Disease Control and Prevention issued new technical guidelines for the industry. On an earnings call, CEO Frank Del Rio said he was perplexed, flabbergasted and outraged that the CDC is requiring ships go through trial sailings, prove healthy-safety measures are in place by applying for a conditional sailing certificate, and agree to inspections by the CDC before any cruise ship resumes sailing. Meeting those new health safety guideline would require more lead time to get a ship ready, which could delay returning to the seas. One analyst said a July restart is not a sure thing. Norwegian is eager to get back on the high seas. Revenues in the first quarters slumped more than 99 percent in the first quarter but bookings for the first-half of next year are already back to levels seen prior to the health crisis. Shares fell more than 7 percent in Thursday trading.