I can second the 3M Skotchkote and 3M 23 Self Bonding Electrical Tape process.
Iv seen this hold well beyond 1000m with no issues.
I would try to use rubber bonding tape if possible. Silicon bonding tape will be fine but the rubber tape will last longer with repeated seawater exposure and bending.
With the Skotchkote and Tape make sure you rough up or chemically etch your cable to ensure that it bonds well. Use multiple layers of tape (usually 3 or more) ensuring each layer overlaps the previous and ensuring Skotchkote is applied between each layer.
As a suggestion only, I would advise testing your Insulation Resistance readings before and after your splice process as it can change if your wires move around during a wrapping process.
HOWEVER … it should be noted that the splice is generally not physically strong. It should be used where the tether is not being repeatedly moved around, like close to the penetrator before the thimble. If the join is constantly moved / bent its likely to have issues. This splicing method is typically used on stationary cables.