Here is another method as per Vyusur which might perform better under smoothing. Rather than start with a square, you can also start with an oval or a cylinder. Here I start with a cylinder with 12 points on loop. Extrude in once to brace the edge loop. Then use Tessalate and join vertexes to center. This yield triangles. No problem. Select two triangles at a time and dissolve (backspace). Now the end of the cylinder is made of quads but the edge loop is braced by an circle. Repeat for other end if closing bottle, or delete other end if bottle is open. Select edge loops of cylinder and shape as desired for bottle length and width.

• according to Tangoalpha, there is a bug in the uvmap preset for cylinders. If so, create seams at the top and bottom edge loops and one vertical seam connecting. Unwrap and arrange islands.
• If top is open, select the whole uvmap, return to modeling room, add thickness, and use small number. If you return to uv room, the new thickness is selected. Use operations, detach selected,and then shift/translate to move duplicate UVs to side if you don't want outer and inner to match (space for a label, for example). (EDIT - sorry, screencaps that I uploaded did not match. I add the other screencaps in posts that follow. Hope it isnt too confusing). Hope it helps.