By "seasons", they mean story arcs, right?
There is some confusion going around about what constitutes a "Season", let me break it down really quick.
A "Season" of a TV show is a set of episodes as ordered for a network. They're called "Seasons" because episodes are ordered to play for seasonal blocks, like "The new set of House episodes for Fox's Summer Season"
Japan doesn't have typical "Seasons" at least not for anime like One Piece. One Piece airs continuously with your occasional one week break, without end. There's never a long break between Seasons.
Toei, when they did their Japanese DVD releases, made up their own Season designations.
Season 1 is East Blue
Season 2 is "Entering the Grand Line", (Apis through Little Garden)
Season 3 is "Enter Chopper at the Winter Island" (Drum)
Season 4 is Alabasta
Season 5 is TV original (Post Alabasta Filler)
Season 6 is Sky Island (Including Jaya)
Season 7 is "Escape! The Naval Fortress and the Foxy Pirate Crew"
Season 8 is Water 7
Season 9 is Enies Lobby (& Ice Hunters)
Season 10 is Thriller Bark
Season 11 is Sabaody
Season 12 is Amazon Lilly and
Season 13 is Impel Down.
! The WB War will probably end up being Season 14
The American seasons for One Piece were dictated exactly the same as they are for normal American TV. US Seasons of One Piece are 52 Episode Seasons as they were ordered for TV by Cartoon Network.
That means, going by the US Broadcast episode numbering (by 4kids) the Seasons are:
Season 1: Ep 1-52 (Ending in the middle of Drum)
Season 2: Ep 53-104 (Ending at the end of Rainbow Mist)
At this point, FUNi picked up and restored the numbering
Season 3: Ep 144-195 (All of Skypiea)
When FUNi put the episodes out on DVD, they had to account for the 39 extra episodes that 4kids had cut out. They could have either split them off into their own Season (Making Skypiea season 4) or stick to the original Season numbering and just slide the episodes in there. They chose the later and decided to make…
Season 1 - East Blue (Ep 1-54)
Season 2 - Baroque Works (Ep 55-143)
Season 3 - Skypiea (Ep 144-195 or 206 depending on if G8 is S3 or S4)
At this point, the show is no longer going to be ordered for TV, so they have no "Official" List to follow beyond wherever the official season end for S3 was, so at this point it is entirely within their power to just make up seasons however they want. I think they'll probably keep splitting them up by Story Arcs as they have been, though Water 7 and Enies Lobby are each a tad too long to make it as one complete season I think. If I were to predict the rest, I'd probably say it'd be like this:
Season 4 - the Davy Back Fight & Water 7
Season 5 - Enies Lobby
Season 6 - Ice Hunters & Thriller Bark
Season 7 - Sabaody, Amazon Lilly and Impel Down
(But again, those are just my predictions)