Building a Pier with Pilings
There is an excellent tutorial on how to build a pier on a beach lot here, but it has one major (IMHO) issue - the dock is unsupported.
The following tutorial shows how to build a pier with supports
1) Decide where you want your pier
2) Set moveObjects on and place columns, starting at the water's edge and advancing down the slope into the sea. The pier is best three squares wide, but could be two. However, it does need to be an odd number of tiles long
3) Starting at the beach end, tile row by row on top of the columns
4) Holding down the shift key, drag stilt decking the width of the pier and in lengths of 3 or 1, leaving a 1-wide gap between sections
5) Fill in the gaps with deck tiles
6) Remove the lower tiles, and then the columns
7) Set moveObjects off
8) On land and in line with one corner of the pier, dig a hole with 4 sets of stairs
9) Remove the stairs and build a 1x1 foundation at the bottom of the hole and place a pillar on top of it
10) Set constrainFloorElevation off
11) Level from the top of the pillar to one corner of the pier
12) Level the rest of the pier
13) Remove the pillar and foundation, fill in the hole and tidy up the beach with terrain paint
14) Set constrainFloorElevation on
15) Add steps
16) Decorate as desired