There are two different Aeotec leak sensor models, one is Zwave and one is Zigbee, but neither uses WiFi.
Both zwave and the zigbee that smartthings uses are “mesh networks“ which means that other devices on the same network can pass along messages for each other. But only mains powered devices can do this, battery powered devices usually don’t because it would use up too much battery power.
So first we need to know the model number of the sensor, and then you probably just need to add one or two repeating devices of the same protocol between where the sensor is and the hub. Many people just use a smart plug because these are inexpensive and easy to find a place for. Just remember zwave repeats only for Z wave and Zigbee repeats only for Zigbee, so we do need to know which model sensor you have.
Basements are often tricky because of cement, metal pipes, and sometimes even insulated ceilings, so the practical range of your sensor may be less than it would be on the next floor.
Start by reading post 11 in the following community FAQ (I will link directly to that post), then after you have read that, go up to the top and read the whole thread and then you should know more about how to lay out your network. 
(The topic title is a clickable link)