Sessile marine organisms, especially marine sponges, represent an important basis for potential development of certain drugs since they are rich in microorganisms and bioactive natural products. Such an example are inhibitors of acetylcholinesterase, which are already widely used for medical purposes. None of the acetylcholinesterase inhibitors from the marine realm are yet commercial products, therefore collecting and analysing marine organisms represent a valuable effort to further discover new natural products and their potential medical use. In order to investigate their pharmacological effects, a set of semisynthetic analogs (MBC) was designed out of the marine natural products isolated from the sponge Geodia barretti. These natural compounds had previously shown inhibitory activity towards acetylcholinesterase. Another set of analogs (Kine) was designed out of bromocyonine isolated from the Mediterranean ascidian Ciona edwardsii. Tested analogs have shown acetylcholinesterase-inhibitory activity. Analyses of Dixon plots have revealed these compounds as reversible competitive inhibitors. None of the semisynthetic compounds were as active as isolated compounds barettin and 8,9-dihydrobarettin. The most active analogs MBC-197 and Kine22 underwent further testing on neuromuscular preparation. In tested concentrations, none of them exerted an action on neuromuscular transmission nor affected membrane potentials.