If someone eats a devil fruit, they can get some power depending on the fruit that they eat. The side effect for eating this fruit is that the user cannot swim.
How about the artificial devil fruits that are made by Dr. Vegapunk? Is it the case that people that eat artificial devil fruits also can't swim and are weakened by the sea?
Yes, artificial devil fruit users suffer from the same weakness has the "natural" devil fruit counterparts: they cannot swim. This was shown in Chapter 795, page 6:
I would like to add something in response to Namikaze Sheena's comment. First of all, to be precise, it is not that devil fruit users cannot swim, but rather they become powerless when their body is submerged in water. As a consequence, they cannot swim or even move at all.
Now, if Vegapunk were to feed a devil fruit to a boat, I think (this is pure speculation) it would not sink: the boat floats because wood is lighter than water, which is a physical property that it would keep even on top of water. It would lose strength, but maybe not completely as we have previously seen Luffy retain control of his body if only a fraction of it is submerged (e.g. on their way to fishman island, Chapter 605, page 11).
EDIT:
On the above picture, we can somewhat see Brook and Momosuke (in dragon's form) drowning. Here is a picture of Momosuke is his Zoan form:
Yes they cannot swim!
This fruit grants the general weaknesses a natural Devil Fruit would, as shown when Momonosuke could not swim.
You can find it on the Wikia: http://onepiece.wikia.com/wiki/Artificial_Devil_Fruit
