The supply of commercial plant biostimulants (PB) for sustainable agriculture is currently very broad but also confusing, as there is little information on their use to mitigate the negative effects of water stress on plants growing in areas of water scarcity. The issue addressed in this article deals with the effects of Amalgerol®, a PB and soil conditioner mainly based on seaweed extracts (SWEs), on the water relations and the growth patterns of mandarin trees grown in pots and their response to a subsequent period of water stress compared with un-treated plants. When the SWE treatment accumulated 75 mL of product, plants exhibited an increase in vegetative growth and higher values of gas exchange rate, with 57% higher substrate microbiological activity than un-treated plants. After this, the irrigation was completely suppressed in all plants until a mean threshold value of −1.6 MPa of midday stem water potential was reached, and it was then reestablished after 7 consecutive days. The un-treated plants showed a higher level of water stress, around 0.4–0.7 MPa, compared to the treated ones, recovering at least three days after irrigation recovery. Furthermore, the presence of mycorrhized roots was 60% higher than un-treated plants, which resulted in greater resistance to water stress. The use of Amalgerol® resulted in a good complement for mineral plant fertilization in semi-arid agrosystems, where water resources are limited, allowing the hardening of citrus nursery plants, which can contribute to their more efficient field transplantation in water scarcity areas.